Handbook of Human Factors in Medical Device Design

Handbook of HUMAN FACTORS in MEDICAL DEVICE DESIGN TAF-ER562X_WINGR-09-0401-C000.indd i 11/9/10 1:53:35 AM TAF-ER5...

Author: Matthew Bret Weinger | Michael E. Wiklund | Daryle Jean Gardner-Bonneau

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Handbook of

HUMAN FACTORS in MEDICAL DEVICE DESIGN

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Handbook of

HUMAN FACTORS in MEDICAL DEVICE DESIGN Edited by

Matthew B. Weinger Michael E. Wiklund Daryle J. Gardner-Bonneau Assistant Editor

Lori M. Kelly

Boca Raton London New York

CRC Press is an imprint of the Taylor & Francis Group, an informa business

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CRC Press Taylor & Francis Group 6000 Broken Sound Parkway NW, Suite 300 Boca Raton, FL 33487-2742 © 2011 by Taylor and Francis Group, LLC CRC Press is an imprint of Taylor & Francis Group, an Informa business No claim to original U.S. Government works Printed in the United States of America on acid-free paper 10 9 8 7 6 5 4 3 2 1 International Standard Book Number-13: 978-1-4200-6351-6 (Ebook-PDF) This book contains information obtained from authentic and highly regarded sources. Reasonable efforts have been made to publish reliable data and information, but the author and publisher cannot assume responsibility for the validity of all materials or the consequences of their use. The authors and publishers have attempted to trace the copyright holders of all material reproduced in this publication and apologize to copyright holders if permission to publish in this form has not been obtained. If any copyright material has not been acknowledged please write and let us know so we may rectify in any future reprint. Except as permitted under U.S. Copyright Law, no part of this book may be reprinted, reproduced, transmitted, or utilized in any form by any electronic, mechanical, or other means, now known or hereafter invented, including photocopying, microfilming, and recording, or in any information storage or retrieval system, without written permission from the publishers. For permission to photocopy or use material electronically from this work, please access www.copyright.com (http://www.copyright.com/) or contact the Copyright Clearance Center, Inc. (CCC), 222 Rosewood Drive, Danvers, MA 01923, 978-750-8400. CCC is a not-for-profit organization that provides licenses and registration for a variety of users. For organizations that have been granted a photocopy license by the CCC, a separate system of payment has been arranged. Trademark Notice: Product or corporate names may be trademarks or registered trademarks, and are used only for identification and explanation without intent to infringe. Visit the Taylor & Francis Web site at http://www.taylorandfrancis.com and the CRC Press Web site at http://www.crcpress.com

Dedication The editors express their gratitude to Peter Carstensen for making this Handbook possible.

After a distinguished career in aerospace, Peter spent over 30 years with the U.S. Food and Drug Administration (FDA), where he introduced the agency to human factors engineering and ultimately served as its human factors team leader. His intelligence, patience, and wit served him well in his role as a human factors advocate within the government and as a convener of and participant in international human factors standards committees. Having retired from the FDA in 2008, Peter is still improving medical device safety and usability through his efforts as a human factors engineering and regulatory compliance consultant. On joining the FDA in the mid-1970s, Peter and his colleagues recognized that medical device mishaps were frequently related to user-interface design shortcomings. Beginning with initiatives to make anesthesia equipment safer through the application of human factors design principles and more recently leading the international community to establish standards for the application of human factors in medical device design, Peter’s tireless work helped give human factors a “seat at the table” in settings ranging from engineering meetings to regulatory reviews. As a result, with support from his FDA colleagues (notably Dick Sawyer, Robert Cangelosi, Ron Kaye, and Michael Mendelson), Peter made a substantial difference in the world, improving the safety of medical devices and likely saving an untold number of patient lives.

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Contents Introduction ..................................................................................................................... ix Michael E. Wiklund Chapter 1 General Principles ...........................................................................................1 Michael E. Wiklund and Matthew B. Weinger Chapter 2 Basic Human Abilities...................................................................................23 Edmond W. Israelski Chapter 3 Environment of Use .......................................................................................63 Pascale Carayon, Ben-Tzion Karsh, Carla J. Alvarado, Matthew B. Weinger, and Michael Wiklund Chapter 4 Anthropometry and Biomechanics ................................................................97 W. Gary Allread and Edmond W. Israelski Chapter 5 Documentation ............................................................................................153 John W. Gwynne III and David A. Kobus Chapter 6 Testing and Evaluation ................................................................................201 Edmond W. Israelski Chapter 7 Controls ......................................................................................................251 Stephen B. Wilcox Chapter 8 Visual Displays ............................................................................................297 William H. Muto and Michael E. Maddox Chapter 9 Connections and Connectors ....................................................................... 351 Joseph F. Dyro Chapter 10 Alarm Design ..............................................................................................397 Stephen B. Wilcox Chapter 11 Software User Interfaces ............................................................................425 Michael E. Wiklund vii

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Chapter 12 Workstations ................................................................................................471 Michael E. Wiklund Chapter 13 Signs, Symbols, and Markings ....................................................................543 Michael J. Kalsher and Michael S. Wogalter Chapter 14 Packaging ....................................................................................................595 Michael E. Maddox and Larry W. Avery Chapter 15 Device Life Cycle ........................................................................................623 Michael E. Maddox and Larry W. Avery Chapter 16 Hand Tool Design ........................................................................................645 Richard Botney, Mary Beth Privitera, Ramon Berguer, and Robert G. Radwin Chapter 17 Mobile Medical Devices..............................................................................715 Richard Stein and Michael E. Wiklund Chapter 18 Home Health Care .......................................................................................747 Daryle J. Gardner-Bonneau Chapter 19 Cross-National and Cross-Cultural Design of Medical Devices .................771 Uvo Hoelscher, Long Liu, Torsten Gruchmann, and Carl Pantiskas Editors ............................................................................................................................795 Contributors ..................................................................................................................797 Index .............................................................................................................................. 805

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Introduction Michael E. Wiklund The editors and contributing authors developed this Handbook to promote the design of safe, effective, and usable medical devices. Broadly speaking, it is a response to the historically high rate of injuries and deaths that have been directly or indirectly caused by medical devices with user-interface design shortcomings*. Following the handbook’s guidance should help medical device developers create user interfaces that are less prone to use error. The resulting devices will be more likely to support the highest quality of medical care while enhancing user productivity.

ABOUT HUMAN FACTORS Readers who are new to human factors should be excited to learn how much the discipline has to offer the medical device industry, health care providers, and patients. By applying knowledge about human characteristics to produce effective interfaces, human factors practitioners help to create medical devices that are easier to learn and use. Specifically, a well-designed user interface enables device users, such as physicians, nurses, therapists, and technicians, to draw on their past experience to use a new device, avoid errors, quickly detect and recover from errors when they occur, feel confident about their decisions and actions, and be physically comfortable. As such, a good user interface can improve safety and the quality of patient care. Conversely, a poorly designed user interface can respond in unexpected ways to user actions, increase workload, induce critical errors that jeopardize patient and clinician safety, and cause fatigue and discomfort. Medical device developers who invest in human factors find that increased user interface quality gives them a competitive advantage in the marketplace because customers prefer their products to other offerings. The investment also meets a standard of care that has been established over several decades as more companies incorporate human factors processes and design practices into their device development process. Moreover, medical device manufacturers are now compelled to invest in human factors by international standards† and the expectations set forth by medical device regulators, such as the U.S. Food and Drug Administration, Health Canada, and Germany’s Federal Institute for Drugs and Medical Devices.

HANDBOOK CONTENT As a prelude to detailed design guidance, Chapters 1 through 6 provide advice on fundamental topics, such as aligning the interactive nature of medical devices to the expected use * Institute of Medicine. To err is human – Building a safer health system (Washington, DC: National Academies Press, 2000) p. 1. † International Electrotechnical Commission. IEC 60601-1-6: Medical Electrical Equipment – Part 1-6: General Requirements for Safety – Collateral Standard: Usability. 2004. ix

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environments ranging from hospitals to ambulances to patients’ homes, drawing on anthropometric and biometric data to ensure that designs match the intended users’ bodies and physical abilities, and conducting usability tests and other evaluations to ensure that devices perform as intended. Chapters 7 through 19 have an applied design focus, offering guidance on the design of specific types of devices (e.g., Chapter 16, “Hand Tool Design”) as well as designing devices for particular use environments (e.g., Chapter 18, “Home Health Care”). Adapted in part from established design standards and conventions, the design guidance also reflects professional judgment extracted from many years of applied analysis and design. Therefore, while much of the guidance is founded on empirical data, it is also based on the contributing authors’ applied design and evaluation experience. Accordingly, readers should exercise their own judgment when applying the design guidance and note that published standards should always take precedence. Although this Handbook’s content is the most current available as of early 2007, other resources should be consulted, as needed, to ensure that the design guidelines remain valid.

RELATIONSHIP TO AAMI STANDARDS The genesis for this Handbook was the work of the Association for the Advancement of Medical Instrumentation’s (AAMI’s) Human Factors Engineering Committee, which produced ANSI/AAMI HE-74-2001, Design Process for the Human Factors Engineering of Medical Devices. This standard provides design process guidance but no specific device design guidelines. The committee recognized the need for a comprehensive resource for human factors design guidance that was specifically tailored to medical devices. One of the committee’s first national standards, ANSI/AAMI HE-48-1993, Human Factors Engineering Guidelines and Preferred Ppractices for the Design of Medical Devices, contained a modest amount of design guidance, much of which was adopted from an existing standard for military equipment (MIL-STD-1472—Human Factor Engineering). The Committee concluded that a subgroup of its members, augmented by outside human factors specialists, should independently produce a handbook that the committee could then draw on to produce an updated national standard. As planned, AAMI’s Human Factors Engineering Committee drew content from this handbook to produce its own design standard for medical device user interfaces (ANSI/ AAMI HE-75-2007, a complement to ANSI/AAMI HE-74-2001). However, there are notable differences between the AAMI’s design standard and this Handbook: • This Handbook includes expanded discussions of design issues, product design case studies, and supporting illustrations. • AAMI’s standard has a terse writing style and explicit organizational structure befitting a standard. • Readers can expect minor differences in the guidance found in the two documents. • This Handbook’s content was not vetted by the AAMI committee, which includes representatives from numerous medical device manufacturers as well as independent medical product design professionals.

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INTENDED AUDIENCE This Handbook should be a valuable resource for the following people involved in the myriad aspects of medical device development, marketing, and support: • • • • • • • • • •

Clinical engineers Design strategists/managers Electrical engineers Graphic designers Human factors engineers/ ergonomists Industrial designers Instructional media developers/ writers Marketing specialists Mechanical engineers Medical practitioners

• Patient safety and medical device researchers • Product liability litigators • Product planners/managers • Regulatory affairs specialists • Quality assurance specialists • Risk analysts/managers • Safety engineers • Software developers • User-interface designers

HOW TO USE THIS HANDBOOK The Handbook can serve several purposes: 1. Medical device developers can convert pertinent design guidelines into product requirements. 2. Medical device evaluators, including test and evaluation personnel and regulators, can set performance criteria for design evaluations (e.g., inspections and usability tests) based on the design guidelines. 3. Human factors advocates can cite the Handbook and specific content to raise their organizations’ user-interface quality standards and secure resources for human factors programs. 4. Students and interested professionals can learn about good human factors design practices. Each chapter is written to stand alone, assuming that people will refer to the subject of interest rather than read the Handbook from start to finish. Therefore, certain topics are intentionally addressed by multiple chapters, ensuring the completeness of technical discussion but leading to some inconsistencies because of varying author perspectives and opinions on design. While each chapter presents its own approach to promoting user-centered design, most share a common organization scheme. Most begin with a general introduction to the selected topic, followed by the presentation of general and special design considerations and then specific, numbered design guidelines. Some chapters include one or more cases studies to instantiate the guidance, and conclude with a listing of resources, literature, and Web site references. Guidelines are numbered sequentially and prefixed with the number of the chapter in which they appear (e.g., the guidelines presented in Chapter 11, “Software User Interface,” are numbered from 11.1 to 11.105). Most guidelines employ the term “should” to promote a desirable design characteristic or performance level. In contrast, design standards usually

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employ the term “shall.” By deliberately using the term “should,” the authors leave room for designer judgment and acknowledge that this Handbook serves an advisory purpose rather than establishing mandates. Chapters include numerous cross references to related guidance found in other chapters. This is especially true of closely related chapters, such as Chapter 7, “Controls,” and Chapter 8, “Visual Displays.” Chapters also include numerous tables and illustrations to further elucidate technical discussions and improve readability.

CLOSING REMARKS Undoubtedly, this Handbook could cover many more topics in the manner of a multivolume encyclopedia. Also, each chapter could expand to cover a wider range of design issues. However, the editors are satisfied that the 19 chapters focus on some of the most important human factors issues facing medical device developers while providing numerous references to other valuable resources. We hope that our readers will appreciate the importance of designing medical devices compatible with human needs and preferences. Enhanced medical device user interfaces are certain to make a positive difference in the lives of caregivers and patients alike.

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1 General Principles Michael E. Wiklund, MS, CHFP; Matthew B. Weinger, MD CONTENTS 1.1 Seek User Input............................................................................................................2 1.1.1 Involve Users Early and Often ..........................................................................2 1.1.2 Refine Designs through Usability Testing ........................................................3 1.2 Establish Design Priorities...........................................................................................4 1.2.1 Err on the Side of Design Simplicity ................................................................4 1.2.2 Ensure Safe Use ................................................................................................4 1.2.3 Ensure Essential Communication .....................................................................5 1.2.4 Anticipate Device Failures................................................................................5 1.2.5 Facilitate Workflow ...........................................................................................5 1.3 Accommodate User Characteristics and Capabilities ..................................................6 1.3.1 Do Not Expect Users to Become Masters.........................................................6 1.3.2 Expect Use Errors .............................................................................................6 1.3.3 Accommodate Diverse Users ............................................................................7 1.3.4 Maximize Accessibility ....................................................................................7 1.3.5 Consider External Factors That Influence Task Performance...........................8 1.4 Accommodate Users’ Needs and Preferences .............................................................8 1.4.1 Accommodate User Preferences up to a Point ..................................................8 1.4.2 Do Not Rely Exclusively on “Thought Leaders” ..............................................9 1.4.3 Enable Users to Set the Pace .............................................................................9 1.5 Establish Realistic Expectations of Users ..................................................................10 1.5.1 Do Not Rely on Training ................................................................................10 1.5.2 Do Not Rely on Instructions for Use...............................................................10 1.5.3 Do Not Rely on Warnings ...............................................................................11 1.5.4 Do Not Rely on Memory ................................................................................11 1.5.5 Avoid Information Overload ...........................................................................11 1.5.6 Do Not Assign Users Tasks That Are Better Suited to Other Users or Devices .......................................................................................................12 1.6 Consider Real-World Demands..................................................................................12 1.6.1 Consider the Context of Use ...........................................................................12 1.6.2 Consider Worst-Case Scenarios ......................................................................13 1.6.3 Make Devices as Rugged as Necessary ..........................................................13 1.6.4 Limit User Workload ......................................................................................14 1.6.5 Consider the Potential for Device Migration into Additional Uses or Use Environments ...........................................................................................15 1

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1.7 Develop Compatible Designs .....................................................................................15 1.7.1 Accommodate Mental Models ......................................................................15 1.7.2 Establish Natural or Conventional Mappings ...............................................15 1.7.3 Follow Industry Conventions and Consensus Standards ..............................17 1.8 Optimize User Interactions ........................................................................................17 1.8.1 Make Devices Error Tolerant ........................................................................17 1.8.2 Fail in a Safe Manner ....................................................................................17 1.8.3 Avoid Physical Strain, Repetitive Motions, and Cumulative Trauma ...........18 1.8.4 Enable Users to Anticipate Future Events .....................................................18 1.8.5 Confirm Important Actions ...........................................................................18 1.8.6 Make Critical Controls Robust and Guard Them .........................................19 1.8.7 Clarify Operational Modes ...........................................................................19 1.8.8 Employ Redundant Coding ...........................................................................20 1.8.9 Design to Prevent User Confusion ................................................................20 1.8.10 Don’t Neglect Device Appeal .......................................................................21 1.9 Summary ...................................................................................................................22 Resources ...........................................................................................................................22 References ..........................................................................................................................22 Although an understanding of detailed human factors guidelines is helpful when designing a medical device, a command of the general design principles of human factors engineering is critical. After all, clinicians and caregivers are usually able to cope with devices that have specific design shortcomings as long as the flaws do not lead to serious use errors or pose insurmountable obstacles to accomplishing a task. In fact, few medical devices are perfect from a user-interface design standpoint. They usually violate one specific guideline or another. It’s another story altogether if a medical device violates a general human factors design principle. Serious violations, such as presenting information at an excessive pace or expecting the user to carefully read an instruction manual before using a device, can render a medical device unsafe or unusable. Accordingly, designers should focus on meeting the high-level design principles before they perfect the details. After all, there is no sense in refining a fundamentally flawed device. In contrast, great products arise from fundamentally correct solutions that are subsequently honed to a state of excellence. This chapter provides an overview of these human factors. Many of these principles will be echoed or built on in subsequent chapters. This chapter presents several high-level design principles intended to help designers produce fundamentally correct user interfaces. For those readers unfamiliar with the human factors design process, reference to ANSI/AAMI HE-74 (2001) may be of value.

1.1 SEEK USER INPUT 1.1.1 INVOLVE USERS EARLY AND OFTEN Medical device users can offer invaluable guidance at each stage of user-interface development. Early in the design process, they can critique existing devices, explain contextual factors that must be accommodated in the design, offer a vision of user interactions with


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the device, and help set usability objectives. As a user-interface design evolves, users can comment on features they like and dislike, describe design attributes that give them trouble, and participate in more formal usability tests (see Chapter 6, “Testing and Evaluation”). Toward the end of the design process, users can help to verify the quality of a near-final design by participating in a usability test of a working prototype. A high level of diverse user involvement throughout the design process helps to ensure that the final device is well suited to the intended users. It also avoids last-minute design modifications to resolve usability problems.

1.1.2 REFINE DESIGNS THROUGH USABILITY TESTING Usability testing is a critical component of the human factors engineering process. Usability testing is a reliable method by which to discern user-interface design issues that could affect safety, efficacy, and satisfaction. Progressive manufacturers often extend testing beyond primary tasks (e.g., using a defibrillator to shock a patient in cardiac arrest) to include setup, storage, maintenance, and even repair tasks. In a typical test session, representative device users perform typical or critical tasks in an appropriate environment, which may range from a conference room to a sophisticated, high-fidelity simulation of the intended clinical care environment (Figure 1.1). The level of test fidelity usually increases as the device progresses from a concept to a refined prototype. Testing early in the design process and then several more times as the design evolves is an effective way to prevent user interaction problems from persisting into the later stages of the design process, a time when effective solutions to problems are more limited and expensive to implement. It is important to find a sample of test participants who accurately reflect the range of user characteristics rather than choosing “thought leaders” who bring special knowledge and motivation to the test. User characteristics that should be considered include physical attributes (i.e., traditional ergonomics), abilities and skills, needs, and psychological attributes. When a device will be used by several distinctly different user groups (e.g., both physicians and patients), tests should be conducted separately with representative participants from each group.

FIGURE 1.1 Clinicians participating in a medical simulation that incorporates several medical devices and a sophisticated mannequin. (From the Center for Experimental Learning and Assessment, Vanderbilt University, Nashville, TN. With permission.)


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1.2 ESTABLISH DESIGN PRIORITIES 1.2.1 ERR ON THE SIDE OF DESIGN SIMPLICITY In medical device design, simpler is usually better. Most medical device users dislike devices equipped with all the “bells and whistles,” especially if the “extras” get in the way of performing basic tasks. Indeed, some medical devices are loaded with features intended to give them a competitive advantage over competing devices. Yet features aimed at enhancing sales can cost a company in terms of customer goodwill if they complicate device operation. Accordingly, developers are well advised to produce devices that focus on the basics and exclude features offering little day-to-day value. The added complexity of “bells and whistles” can interfere with initial ease of use and is usually not worth it. That said, designers have to be careful about eliminating advanced features that offer real value to sophisticated users even if such users represent a small percentage of the overall user population. In such cases, faced with divergent market needs, a manufacturer should consider developing two devices rather than a single, compromised version (Figure 1.2). Similarly, for maintenance tasks, simple is again better. Designers should seek ways to limit the level of skill required to maintain and repair a device as well as the number of steps and the need for special tools.

1.2.2 ENSURE SAFE USE Medical devices should minimize the risk of injury to both users and patients, including physical and psychological injury, during normal and emergency device operation. Applying this principle to a CT scanner, designers would promote design solutions that reduce users’ risk of trauma due to moving parts and of patients feeling claustrophobic as a result of being enclosed in a tight space. Applying the principle to a portable patient

FIGURE 1.2 Two ultrasound scanners (GE Logiq 9 and SonoSite 180Plus System), developed by separate manufacturers, are targeted toward different user populations and use environments. (From http://www.stormoff.com and http://images.google.com. With permission.)


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monitor, designers would avoid placing a heavy instrument on a wheeled pole that is vulnerable to tipping over, which could cause injury to both clinicians and patients, cause property damage, and disrupt the care delivery process. On the other hand, user-interface designers must consider the consequences of dynamic user interactions. For example, in designing a portable glucose monitor for home use, the control–display relationships must be designed to minimize the risk of the diabetic user (who may have a preexisting visual impairment and an acutely abnormal blood sugar) collecting the sample incorrectly or misreading the resulting displayed value. Thus, potential device-induced user harm can either be due to static design characteristics (e.g., mechanical consequences of the physical design) or to use errors during device interaction.

1.2.3 ENSURE ESSENTIAL COMMUNICATION During busy and stressful times, people often must work harder to communicate with each other, and this might lead them to raise their voices, repeat themselves to make sure they were heard, or even grasp someone’s arm to get their attention. Similarly, a well-designed medical device should be capable of reliably communicating critical information, such as a change in a patient’s vital signs that could be life threatening, during busy and stressful moments. Accordingly, designers should employ redundant methods to communicate vital information. Also, where possible, they should provide users with a clear and concise explanation of any problem (including the source) and how to correct it. For example, the device may employ both a sufficiently loud audible alarm and a complementary visual alarm, thereby using two sensory channels to increase the chance of detection. Moreover, the visual alarm might flash to draw attention, and the audible alarm might be set at an attention-getting frequency that some would regard as noxious—a means to motivate users to attend to it. (Note: Standards have taken such needs into account, leading to alarms that some users do find annoying when they are not contextually appropriate; see Chapter 10, “Alarm Design.”) Finally, all designs should be evaluated in the context of the overall use environment, including other devices commonly in use, to ensure that the design solution does not result in unintended consequences, including impaired clinician–patient, clinician– clinician, or clinician–device communication.

1.2.4 ANTICIPATE DEVICE FAILURES Devices will fail. When they do, it is important to communicate the failure to users and, where possible, indicate the cause and recommend appropriate remedial action. This is especially important when a device failure, such as the failure of an air-in-blood detector, places a patient at immediate risk. Ideally, devices will fail safely, but sometimes user intervention is needed to ensure a safe outcome. Therefore, designers should consider the full range of failure modes and develop strategies and detailed user-interface or other solutions for coping with them. As with communicating critical information (see above), it is helpful to communicate both the cause of the device failure and proper remedial or coping actions in clear and concise terms.

1.2.5 FACILITATE WORKFLOW Humans are loathe to change to accommodate new medical devices unless there is a clear benefit in terms of work efficiency or effectiveness. Therefore, designers should take care to


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understand the impact that their device might have on task flow, obviously avoiding negative effects. Additionally, designers should analyze how people will use their device, making sure that the user interface is organized to facilitate urgent, frequent, and critical tasks. For example, in a software interface of a physiological monitor, a dedicated, surface-level control for recording a patient parameter waveform would be a better design than relegating this control to a lower-level screen. Potential device uses should be analyzed formally using techniques such as contextual inquiry, task analysis, or usability testing.

1.3 ACCOMMODATE USER CHARACTERISTICS AND CAPABILITIES 1.3.1 DO NOT EXPECT USERS TO BECOME MASTERS While well-designed devices should have high learnability, do not overestimate the ability of users to master a device’s functions. In reality, most users master just the critical (from their perspective) portions of a device’s functions even if the device is only modestly complex (see Table 1.1). In other words, being practical-minded and often pressed for time, users master just the functions they use frequently. They tend to disregard other device functions until they are forced to deal with them, expecting at that time to draw on their intuition and peer support to operate the device correctly. Accordingly, designers should make infrequently used tasks maximally intuitive (especially if the task is life-critical) because most users will approach them in the same manner as novices.

1.3.2 EXPECT USE ERRORS Many factors can contribute to device use error. Therefore, while maintaining a respectful view of device users, designers should assume that users will err frequently (Beydon et al., 2001; Samore et al., 2004). They should not assume that all users will operate a device with equivalent levels of preparation, attitude, vigilance, and motivation. Rather, a disconcerting proportion of users may have insufficient training, have forgotten their training since the last time they used the device, be fatigued from working long hours (Gaba and Howard, 2002; Weinger and Ancoli-Israel, 2002), have insufficient technical aptitude to interact with complex devices, or be rushed, overworked, or distracted. If anything, designers should overestimate rather than underestimate the chances of a use error. Thus, designers should make the errors obvious, provide the means for rapid recovery, and guide users through the recovery process. TABLE 1.1 Comparison of the Varying Levels of Mastery of Infusion Pump Setup and Operation Tasks Level of Mastery at Performing Specific Tasks Sample Users Nurse X Physician Y Biomedical engineer Z


Determine the Total Volume of IV Fluid

Set up a “Piggyback” Infusion

Change the Battery

High Medium Medium

Medium Low Low

Low Low High

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1.3.3 ACCOMMODATE DIVERSE USERS It is perilous to assume homogeneity in the user population. It is even more perilous for designers to assume that users are just like them. This is why many human factors researchers choose to conduct fieldwork leading to the formulation of personas (also called user profiles) of typical users to guide a design effort. Depending on the device, users may indeed be a comparatively small and specialized population, such as highly trained interventional cardiologists who operate catheterization laboratory equipment. However, over-the-counter devices, such as glucose meters, blood pressure monitors, metered dose inhalers, and infant apnea monitors, will be used by diverse individuals, including the young and old and people with disabilities. In either case, designers should accommodate the needs of users who have different sizes, shapes, physical abilities, intellectual capabilities, experiences, and so on (see Chapter 4, “Anthropometry and Biomechanics,” and Chapter 2, “Basic Human Abilities”). A simple example of accommodating user diversity is designing a surgical tool that can be used comfortably by individuals with either small or large hands (see Chapter 16, “Hand Tool Design”). Other examples include designing a computerized patient data entry screen for people who have extensive computer experience as well as those with relatively little experience or a mammography machine that is accessible to both ambulatory individuals and those in wheelchairs.

1.3.4 MAXIMIZE ACCESSIBILITY The term accessibility has traditionally been associated with architectural features, such as sidewalks, building entrances, and restrooms. Features such as curb cuts, automatic doors, and large restroom stalls equipped with assist bars are products of regulations and political activism that have improved accessibility to public spaces. In recent years, consumer electronic and information technology products have incorporated features to make them more accessible to users with physical or sensory impairments. For example, every federal Web site now describes all figures in the text to accommodate people with visual impairments who use screen readers. Similar improvements can be made to medical devices, making them more usable by people with disabilities. As cited above, imaging devices can be made more accessible to people in wheelchairs and those with limited range of motion or sensory-motor control by addressing their needs during device design (Figure 1.3). The

FIGURE 1.3 Mammography machines can accommodate a seated patient. (From http://qap.sdsu. edu/education/breastcancerreview/Bc_diag/Bccore/photo4d.gif. With permission.)


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FIGURE 1.4 First responders might have difficulty reading a defibrillator display if space constraints on a commuter train require them to place the unit at an acute viewing angle. (From http:// www.sanarena.ch/icons/fotos/Nothilfetag2004/reanimation1.jpg. With permission.)

same may be said of many other types of medical devices, including examination tables and all sorts of diagnostic and therapeutic devices. An example of the latter is a glucose meter that speaks user instructions and meter readings to facilitate use by individuals with visual impairments.

1.3.5 CONSIDER EXTERNAL FACTORS THAT INFLUENCE TASK PERFORMANCE Sometimes, people use medical devices in a relatively isolated manner, such as reprogramming an insulin pump while seated at a desk in a quiet room. However, medical devices are often used in a more dynamic and potentially distracting setting (see Chapter 3, “Environment of Use”). It might be quite noisy. It may be particularly hot (e.g., using devices in the course of a rescue conducted outdoors on a 100°F day). Other people or devices may be vying for the user’s attention. Users may be wearing protective gear (e.g., glasses and gloves) to prevent injury or contamination. When considering the many potential external factors, a given design might be found to be incompatible with some uses, such as a paramedic who is wearing thick gloves and trying to press the right button or to read a display at an acute angle (see Figure 1.4). Analyzing the resulting design trade-offs (e.g., different control element choices might be better under different use conditions) is a core part of effective human factors engineering.

1.4 ACCOMMODATE USERS’ NEEDS AND PREFERENCES 1.4.1 ACCOMMODATE USER PREFERENCES UP TO A POINT Medical device consumers—particularly large hospitals that purchase large lots— often ask manufacturers to customize devices according to their institutions’ needs. Such requests often motivate designers to add configuration options into devices. This


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supports the appropriate goal of making devices adapt to the users rather than the other way around. However, such adaptability brings with it the risk of user interfaces becoming less stable or less predictable or even compromising performance. Someone accustomed to one device’s setup may be confused when encountering what appears to be the same type of device but with a completely different setup. For example, a nurse who works shifts at several different hospitals each week may encounter infusion pumps that look the same but work quite differently, setting the stage for a use error due to negative transference. One solution is to limit user-interface variability where possible by developing optimal solutions for all users and then make the remaining setup differences obvious. Consistency across device models regardless of manufacturer (e.g., all parenteral infusion pumps would have the same controls just like all cars have a steering wheel) could substantially reduce use errors. This approach may be resisted, however, because it asks manufacturers to sacrifice brand identity and potential competitive advantages. Also, manufacturer compliance with de facto industry standards may be compromised by existing patents and licensing agreements. An alternative is to design a device that can be readily set to a particular institution’s or individual’s preferences, thereby accommodating market niches while ensuring that such preferences do not impede typical users. Designers should keep in mind that users are not designers. Thus, although users might express specific needs and preferences or even suggest a detailed design, this input may prove unreliable, undesirable, or unworkable. Accordingly, designers should view themselves as interpreters, taking and prioritizing user input while also applying their own expertise and creativity to produce the best possible designs. Selective user input can be particularly dangerous when care is not taken to rigorously study and evaluate potential design alternatives.

1.4.2 DO NOT RELY EXCLUSIVELY ON “THOUGHT LEADERS” Manufacturers are prone to rely on guidance from accomplished and interested clinicians, individuals also referred to as “thought leaders.” Manufacturers also tend to rely more on clinicians who represent large accounts, the goal being to give extra emphasis to the particular institution’s needs in order to keep their business. Indeed, such individuals can be an excellent source of design guidance, particularly with regard to identifying use needs and preferences. However, their relatively sophisticated viewpoint and capabilities as well as their expanded knowledge of specific design issues and trade-offs lead them to develop biases. Also, such individuals might push a particular design solution harder than is appropriate because of their emotional investment in it or even for the sake of ego gratification. Accordingly, design teams should seek users who are more representative of the typical user to define user needs and preferences as well as to get reactions to designs in progress.

1.4.3 ENABLE USERS TO SET THE PACE Human beings become annoyed when machines set the work pace. Often the pace will be too slow or too fast because of individual performance differences. Moreover, machinepaced tasks do not readily accommodate work stoppages due to interruptions, including


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FIGURE 1.5 These software user interfaces give users control over the pace of work by presenting controls to continue and start tasks. (From http://www.visionchips.com/images/s3.gif and http:// alaris.choc.org/graphics/display_wsk_gdl_lido_new_1b.jpg. With permission.)

emergency situations. Thus, designers should let device users set the work pace by, for example, requiring them to provide input before proceeding to the next step in a procedure (Figure 1.5).

1.5 ESTABLISH REALISTIC EXPECTATIONS OF USERS 1.5.1 DO NOT RELY ON TRAINING Caregivers do not always receive proper training before using a given device. As discussed earlier, clinicians’ work demands leave little time for training and for reading instruction manuals. Also, a new employee or a traveling nurse who is filling a temporary gap in staffing may not have received training before he or she uses a device, institutional policies prohibiting such untrained use notwithstanding. Even when users receive proper training, they may forget what they learned by the time they use the device, especially if the device is used infrequently (e.g., a few times per year). Or caregivers who are familiar with a device might simply forget or be confused about how to perform an uncommon task. Accordingly, medical devices should be designed for intuitive operation even when users are expected to be highly trained. Furthermore, if designers anticipate that a medical device may migrate from clinical use to off-label use, for example, from hospitals to use in people’s homes, it should be designed for intuitive use by laypersons even if it is not initially intended for use by such individuals.

1.5.2 DO NOT RELY ON INSTRUCTIONS FOR USE While some manufacturers produce excellent instructions for use, users might still disregard them in favor of a hands-on demonstration (i.e., an in-service) from a manufacturer’s representative, staff educator, or knowledgeable peer. The instruction manual will often be difficult or impossible for users to access while also using the given device (Figure 1.6). Therefore, designers should not count on users reviewing and absorbing information found only in the instructions for use. While the instructions may describe a device’s theory of operation in a helpful manner or may even be essential to understanding the device’s performance, it is unlikely that many users will read them. In most cases, the caregiver’s workday is too hectic to spend the time to thoroughly read or even skim device instructions. Users may even neglect or overlook instructions for use placed on the device, perhaps in the form of a label or online help system. Many users simply ignore instructions in favor of


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FIGURE 1.6 device.

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User manuals are often stored in a central location rather than with the medical

other learning methods. Therefore, designers should account for the reduced opportunity to inform device users through documentation by making devices as intuitive to operate as possible.

1.5.3 DO NOT RELY ON WARNINGS The presence of many warnings and on-device instructions often indicates user-interface shortcomings (see also Chapter 10, “Alarm Design,” and Chapter 13, “Signs, Symbols, and Markings”). The best way to address a hazard is to eliminate it or design a user interface to guard against it. As such, warnings should be regarded as the last but nonetheless useful step toward preventing a problem, especially problems that can lead to property damage or personal injury. Unfortunately, devices become “papered” with warnings when development teams fail to implement the design changes necessary to correct fundamental design problems. This solution is problematic not only because it leaves fundamental flaws intact but also because the presence of multiple warnings may lead to “warning fatigue,” causing users to be less attentive to any warnings on the device.

1.5.4 DO NOT RELY ON MEMORY People can be forgetful and are often easily distracted. Also, daily life can place excessive burdens on short-term memory. Therefore, designers should not rely on users to remember information, such as a test result or numerical code, to perform a task. Moreover, operational sequences should not require users to remember next steps. It is far better to present users with the crucial information they need to perform a task correctly. It is also helpful to bring required tasks to the user’s attention.

1.5.5 AVOID INFORMATION OVERLOAD Medical devices often flood caregivers with more information than they could possibly use to accomplish associated tasks. The result can be information overload, a condition in which the caregiver cannot receive and process the information fast enough for it to be useful (see Chapter 2, “Basic Human Abilities”). One solution is to provide users with


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information of primary interest at the moment it is needed while allowing them to easily obtain secondary information at a later time. For example, an at-home dialysis patient may usually want to know that “everything is okay” and determine how long before his or her treatment is complete but occasionally opt to check his or her blood pressure, the amount of fluid “taken off,” and other dialysis parameters. Another solution is to preprocess information, relieving the caregiver of the task. For example, a device might graph the relationship between two parameters and indicate minimum and maximum values, thereby performing the work that users otherwise must do in their heads. The key is to simplify information while still ensuring that important contextual information and subtle nuances are retained. Another example is emphasizing values in a list that exceed set limits rather than making users recall the limits and search the entire list for excursions from those limits.

1.5.6 DO NOT ASSIGN USERS TASKS THAT ARE BETTER SUITED TO OTHER USERS OR DEVICES Assign users and machines the tasks they are best qualified to perform and make sure that the distribution ensures safety and user satisfaction. One effective way to reduce workload is to let devices perform the functions they do best rather than give people the extra work (see Chapter 2, “Basic Human Abilities”). For example, a device is usually better at monitoring a steady-state process for unusual events, a task that is eventually fatiguing for most people and leads to reduced vigilance. Computer-based devices easily retain information that users may forget. A robotic device can hold an instrument steadily in a precise position for long periods of time. On the other hand, some functions (e.g., complex problem solving or pattern recognition) might be better performed by people and should not be quickly delegated to technology. Designers should automate tasks skillfully to avoid unintended consequences on overall user performance. Even if devices might better perform some tasks, designers should be cautious about shifting so many functions to the device that the users lose their awareness of the current situation as well as their ability to respond to emergencies. People generally prefer to be actors rather than observers in a process, except when the required actions are tedious or fatiguing, divert their attention from more important tasks, or are clearly performed better by machines.

1.6 CONSIDER REAL-WORLD DEMANDS 1.6.1 CONSIDER THE CONTEXT OF USE There is a tendency to create designs that work well for trained users who focus their full attention on operating the device in a quiet environment, such as the previously cited example of reprogramming an insulin pump at a desk in a quiet room. But medical devices are commonly used by diverse users in several different use environments. In addition, medical devices are sometimes used by distracted, fatigued, and/or marginally trained individuals. Clinicians often work in chaotic environments and need to focus more attention on their patients than their equipment. These real-world conditions can have a significant effect on a user’s interactions with a medical device, masking audible signals and interfering with concentration, for example. Designers need to consider realistic use conditions. The first step is to learn about users through field research conducted in a manner that does not appreciably influence the way people are working. Researchers often discover that medical


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devices are in the periphery of the users’ minds, particularly devices that function in a relatively autonomous manner, requiring only occasional checking. Later during device design, the impact of a new device on the ability of users to deliver quality care as well as its ability to work well with other devices should be tested. Well-designed medical devices tend to fit naturally into the home or workplace, garnering favorable reviews from users. Flawed devices tend to draw end users’ dissatisfaction, generating complaints about how they interfere with the normal workflow, and require too much attention.

1.6.2 CONSIDER WORST-CASE SCENARIOS In the normal course of device development, mechanical engineers purposely drop devices to the floor or shake them for hours in a test chamber to see if and how they break. Userinterface designers need to perform equivalent tests of their designs. Thus, user interfaces must be subjected to worst-case scenarios to see if and how they fail. The goal is to see how well an untrained or minimally trained user performs when asked to operate an unfamiliar device; to observe people using a device under harsh environmental conditions, such as at night in a moving ambulance (Figure 1.7) or helicopter; and to see what happens when people are under time pressure to perform a critical and difficult task. By stressing the user interface, designers can learn how to make the device more effective in the real world. Note that minimizing the risk of faulty design or use errors causing human injury requires an integrated approach that includes rigorous design practices, usability testing, and risk analysis.

1.6.3 MAKE DEVICES AS RUGGED AS NECESSARY Some medical devices take a beating during their useful life, which may span 10 to 20 years of daily use, and, just like mechanical components, user-interface components should

FIGURE 1.7 Paramedics use myriad medical devices in a moving vehicle that may complicate device-related tasks, such as reading a display and operating controls. (Shutterstock image 31634293.)


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FIGURE 1.8 Patient transport device has rugged design to be specifically used on stairs over many years. (From http://www.ems.stryker.com/detail.jsp?id=6 and Strykker 2005 Catalog, ems_2005_ catalog.pdf.)

be designed to last this long (see Figure 1.8). Heavily used devices that are subject to jostling and impacts require heavier-duty user-interface components. This means selecting switches that are unlikely to break even if struck with considerable force, screens that will not crack if elbowed or bumped, and labels that will not become unreadable after years of scrubbing with antiseptic solutions. The designer will need to balance the need for ruggedness against other design goals, such as ease of switch actuation and avoiding display parallax.

1.6.4 LIMIT USER WORKLOAD Caregivers are often overworked, enduring vigorous 12-hour or longer shifts, sometimes several days in a row. This explains why some caregivers actively or passively reject medical devices that create too much mental or physical work, as discussed earlier. Clinicians want to focus on their patients, not on distracting technology. Caregivers will seek shortcuts and work-arounds if a device distracts them from more important tasks even if the work-saving strategies are strongly discouraged by their institution. Patients who use medical devices are similarly disinterested in investing undue time or effort into device use given the many other demands and interests in their lives. Therefore, designers should pursue opportunities to reduce the time required to learn how to use and operate devices. For example, a high-quality video complemented by a quick reference card might shorten the time required to learn to operate a ventilator. As another example, a point-of-care blood gas analyzer might allow a clinician to commence with data entry tasks while the device completes a calibration check or prepares to analyze a sample.


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1.6.5 CONSIDER THE POTENTIAL FOR DEVICE MIGRATION INTO ADDITIONAL USES OR USE ENVIRONMENTS It is common for medical devices to migrate from sophisticated medical settings, such as an intensive care unit, to less sophisticated settings, such as inpatient wards, outpatient clinics, and patients’ homes (see Chapter 18, “Home Health Care”). Or a device intended for use on pediatric patients might ultimately be used on adults, such as small women. In some cases, the device manufacturer might not have anticipated such migration or considered the needs of the new user population in the original design efforts. This can lead to problems for both the manufacturer and new users. Consider the case of the infusion pump designed for use in the hospital but used by the parent of a sick child at home, a so-called off-label use. Lacking sufficient medical knowledge or training on how to use the device properly, the parent might experience a use error, leading to a tragic loss of life. In turn, this might lead to a costly lawsuit against the manufacturer. Or consider the case in which a pulse oximeter is used as a respiratory monitor in patients receiving narcotic pain medications at home, but in the presence of supplemental oxygen, the monitor does not alarm until the patient has stopped breathing, leading to an adverse outcome. Such scenarios underscore the value of anticipating alternative uses of medical devices. After all, manufacturers are probably better off protecting users against hazards associated with predictable, unintended device uses rather than reacting to a product liability or personal injury claim.

1.7 DEVELOP COMPATIBLE DESIGNS 1.7.1 ACCOMMODATE MENTAL MODELS People frequently have an established mental model or “big picture” in mind when they use a new device. Usually, their mental model is based on previous experience using similar devices. For example, users may expect certain controls to function in a particular manner and be surprised by a control that functions differently. Or they may be accustomed to configuring the device the way they were taught in nursing school, only to find that a new device requires an alternative approach. Such incompatibilities can make a device harder to learn to use and can induce errors even among experienced users who may unconsciously fall into an old use pattern (sometimes called negative transfer of training). Therefore, designers need to take care to identify established mental models and accommodate them where possible. When the need for change exists, designers are often better off making major rather than minor changes so that the difference is more readily apparent. Also, designers should provide “affordances” that help users form an accurate mental model of how a device works. Affordances might include organizing the user interface according to a simple “metaphor,” effective use of labels, clear and redundant feedback in response to user inputs, helpful warning messages, and a quick reference card that emphasizes how a device is different from other similar devices.

1.7.2 ESTABLISH NATURAL OR CONVENTIONAL MAPPINGS When people associate an action with a design element, such as turning a knob clockwise, they are mapping. An example of natural mapping is squeezing the bag on an anesthesia machine to fill a patient’s lungs with air; the action and outcome are largely self-evident


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FIGURE 1.9 The axial and left–right alignment of this power injector’s piston controls (top of photo) maps naturally to the movement of the pistons (bottom of photo).

(see Figures 1.9 and 1.10 for other examples of mapping). Turning a knob on an anesthesia machine clockwise to increase the rate of gas flow is an example of a conventional mapping; while it may not be as self-evident to a naive individual, experienced anesthesia providers will perform the task in an automatic, subconscious manner. When mappings are natural or conventional, users will find the associated devices more intuitive to operate. Incorrect or

Head Neck Chest Abdomen

Arm

Hand Thigh Calf Misc

Foot

FIGURE 1.10 This power injector’s software screen (unrelated to the power injector shown in Figure 1.9) provides users with an intuitive means of configuring the device for scanning specific body parts by mapping control elements to corresponding body parts. (From EZEM [design] and Wiklund, 1995 [photo].)


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unconventional mappings may cause users to take longer to learn to use a device or increase use errors. The challenge for designers is to establish effective mappings when the natural or conventional one that has emerged over time departs from a given company’s established practice. Or the designer might learn that conventional mappings differ among user populations (e.g., France, Japan, and the United States). In such cases, it might be appropriate to customize devices to the given market or develop optimal mapping through a rigorous process of design iteration and usability testing. More information on how to design interface elements to be more intuitive is provided in other sections of this handbook (e.g., Chapter 7, “Controls,” Chapter 8, “Visual Displays,” and Chapter 12, “Workstations”).

1.7.3 FOLLOW INDUSTRY CONVENTIONS AND CONSENSUS STANDARDS Manufacturers often seek ways to make their devices stand out from competitors’ devices, thereby fortifying their brand identity and competitiveness. On the other hand, device users value consistency, particularly with regard to operational characteristics. When a new device works like similar devices, including consumer products such as cellular phones and word processing software, users have less to learn about the new device and can apply past experience more readily. Human factors specialists call this positive transfer. Therefore, designers should not diverge substantially from conventional design practice or industry standards unless there is a compelling reason to do so. There can be very good reasons to deviate, such as a demonstrable increase in design intuitiveness, task efficiency, or error prevention. In fact, to foster innovation and continual design improvements, designers are encouraged to challenge de facto conventions and incorporate new designs if there is good evidence that the alternative will lead to better user performance. On the other hand, making a device seem different just for the sake of being different is a poor practice. For example, designers should not diverge from the standard color codes for alarm signals simply because they think that a magenta-colored alarm indicator is more conspicuous than a red one or the company’s branding scheme dictates use of the nonstandard color. American users have learned that high-priority visual alarms are red, for example, and expect all devices to adopt this convention.

1.8 OPTIMIZE USER INTERACTIONS 1.8.1 MAKE DEVICES ERROR TOLERANT As stated previously, devices fail, and users make errors. Consistent with modern principles of resilience engineering, device designs should be tolerant of error to minimize harm to users or patients. Design approaches to accomplish error tolerance include failing safe (see below), considering the device in the overall context of use, providing more information about the implications of specific use actions, making errors or unwanted deviations more visible to users, making potential risks more visible to users, and facilitating error recovery.

1.8.2 FAIL IN A SAFE MANNER A basic engineering design principle is to fail safely. Consider the household example of an iron that shuts itself off if the user fails to do so, possibly preventing a fire. For medical


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devices, failing in a safe manner is important because patient lives are at stake. The concept of failing safely can be extended from electromechanical failures to use errors. For example, an infusion device might be designed to prevent users from setting an unsafe level of drug delivery. A laser treatment device should not be able to be activated if the emergency stop control is inoperative.

1.8.3 AVOID PHYSICAL STRAIN, REPETITIVE MOTIONS, AND CUMULATIVE TRAUMA The repetitious nature of many medical procedures, such as firmly squeezing and releasing a surgical stapler, puts caregivers at risk for repetitive motion or cumulative trauma disorders (see Chapter 16, “Hand Tool Design”). Therefore, designers should seek opportunities to reduce the number of repetitive actions required to operate a given device (see Chapter 4, “Anthropometry and Biomechanics”). They should also keep manually applied forces to a minimum, eliminate pressure points between devices and users, and enable users to maintain neutral joint positions. Moreover, they should limit the amount of time users are required to apply a constant force (e.g., continuously squeezing the handles on a grasping tool) even if the force is relatively small. Often, a relatively simple design change will achieve these goals, protecting users from injury.

1.8.4 ENABLE USERS TO ANTICIPATE FUTURE EVENTS To provide the best patient care, caregivers sometimes need to predict the most likely future course of disease manifestations and therapeutic interventions. In other words, caregivers try to figure out what is about to happen rather than simply reacting to what has happened or is happening at the moment. This is especially true in situations in which a caregiver is delivering a therapy that can have a dramatic effect on the patient’s physiological state, such as the intravenous delivery of a blood pressure medication. Where possible, designs should enable caregivers to “see ahead.” For example, a monitoring device might present historical parametric values as well as values forecasted for the next 5, 10, and 30 minutes.

1.8.5 CONFIRM IMPORTANT ACTIONS In the medical arena, confirmation messages may serve an important or even critical purpose, considering that some actions are irreversible and could lead to patient injury (Figure 1.11).

FIGURE 1.11

Sample confirmation message.


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Therefore, even though some users may regard confirmation messages as a hindrance—a wasted extra step—the benefit of such messages often outweighs the annoyance they cause. However, the need for users to perform tasks efficiently should not be underestimated when weighing decisions about which actions require confirmation. Thus, the benefits of confirmation messages should be demonstrated through user testing, ensuring that they do not replace one problem with another, such as users confirming their actions without thinking about it (i.e., performing tasks in a rote manner).

1.8.6 MAKE CRITICAL CONTROLS ROBUST AND GUARD THEM Certain medical devices may be exposed to rough handling, particularly when they are used outdoors or during emergency responses (e.g., patient resuscitation procedures or “codes”). Medical devices are subject to being dropped or bumped. Therefore, the user interface needs to be designed to prevent accidental actuation of critical controls. Incidental contact with a device’s front panel should not, for example, deactivate the device or alter a critical control setting. This is why many medical devices have physically guarded power buttons (Figure 1.12) and require the user to confirm critical adjustments (see Chapter 7, “Controls”). If a critical control fails, an alternative means of control should be provided. For example, in the case of a damaged pump stop switch, a mechanical means of ceasing pump action should be provided (and be readily identifiable by the user).

1.8.7 CLARIFY OPERATIONAL MODES One way that designers seek to simplify medical devices is to incorporate multiple operational “modes.” In principle, this approach is a sensible way to facilitate context-specific tasks and to limit users’ exposure to non-relevant device features. However, problems can arise when the user enters the wrong mode and does not realize it (often called mode error). It would be problematic, for example, if an anesthesiologist were monitoring an adult patient

FIGURE 1.12 (See color insert following page 564.) An emergency stop button on a scanner is recessed to prevent inadvertent actuation. Large size, red color, symbolic label, shape, and recessing also provide redundant means of differentiating this control from others.


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using a monitor placed in “demonstration” mode (in which the numbers displayed are normal and unchanging) or had the anesthesia workstation’s software user interface set for a pediatric case. Accordingly, designers should make operational modes and their characteristics readily apparent.

1.8.8 EMPLOY REDUNDANT CODING Redundant coding of displays and controls can be a powerful way to ensure reliable device operation (see Chapter 7, “Controls,” and Chapter 8, “Visual Displays”). The concern is that a user who may be fatigued or distracted might actuate the wrong control or mistake one display value for another (e.g., a “1” for a “7”). These kinds of use errors are less likely if displays and controls employ more than one means of coding. Coding options include varying the user-interface element’s size, shape, color, texture, or placement. For example, anesthesia machines use redundant coding (knob color, shape, and texture) to ensure that caregivers turn the correct knob to increase the flow of 100% oxygen rather than air (see Chapter 12, “Workstations”).

1.8.9 DESIGN TO PREVENT USER CONFUSION While considering the need for making devices compatible, as discussed elsewhere, also consider when it is appropriate to make devices distinct. For example, it is critical to distinguish power cable receptacles from sensor cable receptacles, thereby avoiding circumstances in which a user might plug a patient sensor lead into an AC outlet and shock the patient (Figure 1.13). Devices and elements thereof may be distinguished using the coding methods described above. In the case of plugs and receptacles, size and shape coding is particularly appropriate, making it impossible to fit a given lead into the wrong port. This principle of using a physical constraint to dissuade undesirable user actions applies to controls, connectors, and other design elements.

FIGURE 1.13 Monitor’s sensor leads are incompatible with AC power receptacles. (From Wiklund, M., Medical Device and Equipment Design: Usability Engineering and Ergonomics, Interpharm Press, Boca Raton, FL, 1995.)


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1.8.10 DON’T NEGLECT DEVICE APPEAL It is important to recognize that human factors in medical device design are not just about achieving safe and effective task performance. They are also about satisfying user needs, which includes making medical devices pleasing to use. Devices that are easy to use as well as appealing to view and touch will engender greater user satisfaction. One payoff from making devices appealing is that patients—and particularly children—may find them less frightening. Moreover, users may be more motivated to learn how to use appealing devices properly. Added appeal may also lead to increased user vigilance and job satisfaction. For example, a user may pay closer attention to a display with a pleasing appearance that also draws attention to important information as opposed to one that has a garish appearance that draws attention to less important information (see Chapter 11, “Software User Interfaces”). Devices designed to provide an impression of quality can inspire greater user confidence (and even pride of ownership). A user may be drawn to a portable patient monitor because of design qualities (e.g., an enclosure that looks attractive and easy to handle) that extend beyond functionality to boost appeal. The same can be said of tools that look comfortable to hold (Figure 1.14). However, medical devices intended for use in the home should not look like toys; otherwise, they might attract children to play with them.

FIGURE 1.14 Close attention to visual and tactile design considerations, such as rounded surfaces, distinguishable controls, and other styling cues, contribute to a device’s usability and appeal and earned these sinus cavity surgery and eye examination devices Medical Design Excellence Awards. (From http://www.gyrus-ent.com/health/diego/rhinologyDiego.htm and http://www.welchallyn. com/medical/products/catalog/detail.asp?ID=29365. With permission.)


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1.9 SUMMARY The general considerations (i.e., principles) discussed above are just a fraction of the broadlevel factors, albeit some of the more important ones, to consider when designing a medical device’s user interface. Accordingly, readers should regard these as a starting point and supplement them with additional considerations presented in the following sections of this handbook as well as other reference documents.

RESOURCES Jacko, J. A. and Sears, A. (2003). The Human-Computer Interaction Handbook: Fundamentals, Evolving Technologies and Emerging Applications. Mahwah, NJ: Lawrence Erlbaum Associates. National Aeronautics and Space Administration. (1989). Man-Systems Integration Standards. NASA-STD-3000a. Houston: Lyndon B. Johnson Space Center. Nielson, J. (1993). Usability Engineering. Boston: Academic Press. Norman, D. (1988). The Design of Everyday Things. New York: Basic Books. Rouse, W. B. (1991). Design for Success: A Human-Centered Approach to Designing Successful Products and Systems. New York: Wiley-Interscience. Salvendy, G. (Ed.). (2006). Handbook of Human Factors and Ergonomics (3rd ed.). New York: John Wiley & Sons. Sanders, M. S. and McCormick, E. J. (1993). Human Factors in Engineering and Design (8th ed.). New York: McGraw-Hill. Sawyer, D. (1996). Do it by Design: An Introduction to Human Factors in Medical Devices. Washington, DC: U.S. Department of Health and Human Services, Food and Drug Administration. U.S. Department of Defense. (1996). Human Engineering Design Criteria for Military Systems, Equipment, and Facilities. MIL-STD-1472F. Washington, DC: U.S. Department of Defense. Wickens, C. and Holland, J. (2000). Engineering Psychology and Human Performance (3rd ed.). New York: Prentice Hall. Wiklund, M. (Ed.). (1995). Medical Device and Equipment Design: Usability Engineering and Ergonomics. Boca Raton, FL: Interpharm Press. Wiklund, M. and Wilcox, S. (2005). Designing Usability into Medical Devices. Boca Raton, FL: Interpharm Press. Woodson, W. E., Tilman, B., and Tilman, P. (1992). Human Factors Design Handbook: Information and Guidelines for the Design of Systems, Facilities, Equipment, and Products for Human Use (2nd ed.). New York: McGraw-Hill.

REFERENCES American National Standards Institute/Association for the Advancement of Medical Instrumentation (ANSI/AAMI). (2001). Human Factors Design Process for Medical Devices. ANSI/AAMI HE-74-2001. Arlington, VA: Association for the Advancement of Medical Instrumentation. Beydon, L., Conreux, F., Le Gall, R., Safran, D., Cazalaa, J. B., et al. (2001). Analysis of the French health ministry’s national register of incidents involving medical devices in anaesthesia and intensive care. British Journal of Anaesthesia 86:382–87. Gaba, D. M. and Howard, S. K. (2002). Patient safety: Fatigue among clinicians and the safety of patients. New England Journal of Medicine 347:1249–55. Samore, M. H., Evans, R. S., Lassen, A., Gould, P., Lloyd J., Gardner, R. M., et al. (2004). Surveillance of medical device-related hazards and adverse events in hospitalized patients. Journal of the American Medical Association 291:325–34. Weinger, M. B. and Ancoli-Israel, S. (2002). Sleep deprivation and clinical performance. Journal of the American Medical Association 287:955–57.


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2 Basic Human Abilities Edmond W. Israelski, PhD CONTENTS 2.1 Sensory and Perceptual Abilities ...............................................................................25 2.1.1 Vision ..............................................................................................................25 2.1.1.1 Threshold of Seeing ........................................................................25 2.1.1.2 Visual Acuity ..................................................................................26 2.1.1.3 Visual Angle....................................................................................27 2.1.1.4 Minimum Visual Angle ..................................................................27 2.1.1.5 Dynamic versus Static Acuity .........................................................29 2.1.1.6 Accommodation (Focusing Abilities) .............................................29 2.1.1.7 Visual Field .....................................................................................29 2.1.1.8 Color Vision ....................................................................................29 2.1.1.9 Color Vision Deficiencies ................................................................32 2.1.1.10 Color Discrimination Recommendations ........................................33 2.1.1.11 Recommendations for Printed Colors .............................................34 2.1.1.12 Recommendations for Colored Lights.............................................34 2.1.1.13 Recommendations for Color Combinations (Legibility and Visibility) ..................................................................................35 2.1.1.14 Dark Adaptation ..............................................................................36 2.1.2 Visual Perception ............................................................................................37 2.1.2.1 Distance and Perceived Size ...........................................................37 2.1.2.2 True Object Size ..............................................................................38 2.1.2.3 Common Visual Illusions ................................................................38 2.1.2.4 Perception of Motion .......................................................................39 2.1.2.5 Flickering Lights .............................................................................39 2.1.2.6 Photosensitive Epilepsy ...................................................................39 2.1.3 Auditory Perception ........................................................................................40 2.1.3.1 Loudness Measurements .................................................................40 2.1.3.2 Relationship of Phones to Sones .....................................................41 2.1.3.3 Loudness (Sones) Calculation for Complex Sounds ........................42 2.1.3.4 Pitch Measurement ..........................................................................42 2.1.3.5 Differential Hearing Thresholds .....................................................42 2.1.3.6 Effects of Aging on Hearing Sensitivity .........................................43 2.1.4 Other Sensory Modalities ...............................................................................45 2.1.4.1 Skin (Somesthetic) Senses ...............................................................45 2.1.4.2 Muscle Sense ...................................................................................46 23


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24


2.1.4.3 Sense of Balance .............................................................................46 2.1.4.4 Chemical Senses .............................................................................46 2.1.5 Other Perceptual Abilities...............................................................................46 2.1.5.1 Estimation of Time Intervals...........................................................46 2.1.5.2 Estimation of Other Physical Quantities .........................................46 2.2 Human Information Processing .................................................................................48 2.2.1 Limitations on Information Processing Abilities ............................................48 2.2.1.1 Channel Capacity ............................................................................48 2.2.1.2 Attention..........................................................................................49 2.2.1.3 Vigilance (Sustained Attention) ......................................................49 2.2.2 Speed of Information Processing ....................................................................49 2.2.2.1 Reaction Time .................................................................................49 2.2.2.2 Speed versus Accuracy....................................................................51 2.2.2.3 Human Memory ..............................................................................51 2.2.2.4 Working Memory ............................................................................51 2.2.2.5 Long-Term Memory ........................................................................53 2.2.2.6 Estimation and Decision-Making Abilities .....................................53 2.3 Human Response Capabilities ...................................................................................54 2.3.1 Speed of Movement ........................................................................................54 2.3.2 Principles of Motion Economy .......................................................................56 2.3.3 Speech Attributes ............................................................................................57 2.3.3.1 Loudness Levels of Speech .............................................................57 2.3.3.2 Frequency Characteristics of Speech ..............................................58 2.4 Human versus Machine Capabilities .........................................................................59 Resources ...........................................................................................................................60 References ..........................................................................................................................60 This chapter presents a brief overview of basic human skills and abilities that will aid the designer of medical devices to better understand the guidance provided in subsequent chapters in this book. These basic human skills and abilities and their interrelationships are shown in Figure 2.1. The organization of this chapter follows the flow of how humans sense, perceive, process, and respond to the world around them, as shown in the figure that portrays the basic inputs and outputs of the human information processing system. Specifically, the following areas of basic human capabilities and corresponding limitations are covered: vision, visual perception, audition (or hearing), sensation, information processing, human response capabilities, and human versus machine trade-offs. Many specific design recommendations result from knowledge of basic human skills and abilities, and these recommendations are covered in specific chapters in this book (see Chapter 8, Stimuli

Sensory processing

Input

FIGURE 2.1

Response

Perception

Information processing

Response processing

Output

Organization of basic human skills and abilities.


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Basic Human Abilities

25

“Visual Displays”; Chapter 10, “Alarm Design”; and Chapter 16, “Hand Tool Design”). Design guidance from this chapter is noted by numbered guidelines.

2.1 SENSORY AND PERCEPTUAL ABILITIES This section covers basic human sensory and perceptual abilities in the areas of vision, hearing, touch, balance, and perceptual estimation. Individual differences in perceptual abilities are quite large and are due not only to inherent physiological factors but also differences in experience, motivation, and preconceived ideas about incoming sensory information, sometimes called one’s “psychological set” or “expectations.” It should be noted that unless specifically mentioned, much of the following data are based on relatively young humans without major disabilities. Designers must be careful when applying data from this chapter if their intended users include special populations, such as the elderly or disabled. In that case, information provided in Chapter 18, “Home Health Care,” may be more useful.

2.1.1 VISION The human visual sensory system is quite complex. In this section, information is presented on visual thresholds for seeing, visual acuity (both static and dynamic), visual angles, accommodation or focusing, visual field, color vision, and dark adaptation. 2.1.1.1 Threshold of Seeing The sensitivity of the human visual system covers a wide range, as does the visual threshold (i.e., the minimum light level in which an object can be visually identified) under various ambient lighting conditions (Figure 2.2; Van Cott and Kinkade, 1972). Rod vision comes from visual receptors found on the back of the eye, the retina, that are most sensitive under 100,000 Upper limit of visual tolerance 10,000 1,000

Approximate luminance level (brightness) in ft. lamberts

100

Cone vision only

Average earth on a clear day Average earth on a cloudy day

10

White paper in good reading light

1

White paper 1 ft. from std. candle

0.1 0.01 0.001 0.0001 0.00001

0.000001

FIGURE 2.2

Fresh snow on a clear day

Snow in full moon

Rod and cone vision

Average earth in full moon

Snow in starlight Grass in starlight

Rod vision only

Absolute threshold of seeing

Threshold of seeing and luminance levels of various objects.


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26


low light conditions to shades of black and white (e.g., a patient room during sleeping hours). Cone vision comes from visual receptors that are sensitive to color and operate best under higher light levels (e.g., in an operating room). 2.1.1.2 Visual Acuity A number of measures of visual acuity exist: • • • • •

Minimum distinguishable (detection of detail in an arbitrary test target) Minimum perceptible (detection of a spot, e.g., on a radiograph) Minimum separable (detection of a gap between parts of a target) Stereoscopic acuity (detection of depth for a three-dimensional target) Vernier acuity (detection of lateral displacement of one line from another)

These measurements of acuity all apply to static or stationary objects. In addition, there is dynamic visual acuity of the smallest detail that can be detected for a moving target. Visual acuity can be affected by a variety of factors (Table 2.1). TABLE 2.1 Factors That Affect Visual Acuity Factor

Positive Example

Negative Example

Amount and kind of illumination

Bright operating room light

Viewing time

Momentary occlusion message on IV Long time period for viewing pulse pump screen oximeter readings Light-colored numbers showing pulse Yellow trace lines of respiratory rate rate on a dark-background patient on a white-background patient monitor monitor Large font indicating on/off for a Small print on a catheter package ventilator power switch label indicating French size

Object contrast with background Object size (visual angle subtended by the object at the eye) Object color Direction of viewing (position of the image on the retina)

Glare from outside sunlight on IV pump screen

Bright red flashing alarm on an Lettering in pale pastel colors enteral pump indicating it is empty indicating length of a nasogastric tube Patient monitor screen placed at a PCA pump placed on the bottom of 45-degree angle above a patient’s bed an IV pole at knee level

Movement of the object or viewer

Stationary IV pump screen

Heart monitor vibrating from the motion in an ambulance

Accommodation or focusing abilities of the viewer’s visual system

Large-screen monitor (over 17 inches) for an ultrasound machine

Fatigued operator of a small-screen portable patient monitor

Optical alignment of both eyes or convergence abilities

Surgeon being able to accurately judge the depth of cutting with a scalpel

Surgeon attempting to judge distance from a laparoscopic pincer by looking at a video monitor

Dark adaptation

Ambulance driver reading red gauges while driving at night

Trying to find a central line port in a darkened patient room after entering from a brightly lit hallway


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27

7 mm Visual angle A

Size of the object So

tan A

Distance of object do or A (minutes of arc)

(57.3) 60 So do

if A

So do 10°

FIGURE 2.3 Calculation of visual angle A in minutes of arc when object size and distance are known.

2.1.1.3 Visual Angle The visual angle subtended on the retina at the back of the eye can be calculated using the formula and related diagram in Figure 2.3 (Cornsweet, 1970). In the diagram, the apex of the triangle is assumed to be 7 mm behind the foremost point of the cornea. Examples of the visual angle A cast by some common objects at a given distance do are shown in Table 2.2 (Cornsweet, 1970). 2.1.1.4 Minimum Visual Angle Minimum visual angle is the value of the visual angle cast on the retina for the following types of limiting conditions (Dreyfuss, 1966): • Minimum perceptible visual angle is approximately 1 second of a degree for a thin wire against bright sky. (Visible stars may subtend an angle as low as 0.056 second.) • Preferred angle for reading English text is 20 to 22 minutes of arc. Marginally acceptable angles range from 16 to 18 minutes of arc, with 12 minutes considered the threshold of readability. Guideline 2.1: Minimum Type Size Type size should not be less than 3 points when read at 14 inches under the most favorable lighting conditions (1 point = 1/72, or 0.01384 inch). See Table 2.3 for details.

TABLE 2.2 Visual Angle for Common Objects Object Sun Moon Quarter Quarter Quarter Lowercase pica-type letter

Distance (do)

Visual Angle (A)

93,000,000 miles 240,000 miles Arm’s length (70 cm) 90 yards 3 miles Reading distance (40 cm)

30 minutes 30 minutes 2 degrees 1 minute 1 second 13 minutes

1 degree = 60 minutes of arc; 1 minute = 60 seconds of arc; tan 1 second = 0.0000048; tan 1 minute = 0.00029. For small angles, the tangent of an angle varies linearly with the size of the angle (e.g., tan 10 minutes = 10 × tan 1 minute).


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TABLE 2.3 Recommended Character Sizes and Corresponding Font Sizes for Various Reading Distances Character Height vs. Reading Distance Character Heighta (in.)

Reading Distance (in.)

Visual Angle (minutes of arc)

Font Sizeb

Preferred (Upper Bound) 0.102

16

22

Actual font size 7.5

0.154

24

22

Actual font size 11

0.230 0.768 1.152

36 120 180

22 22 22

Font 55 Font 72

0.093 0.140

16 24

0.209 0.698 1.047

36 120 180

0.084 0.126

16 24

0.188 0.628 0.942

36 120 180

0.074 0.112

16 24

0.168 0.558 0.838

36 120 180

16 16 16

0.056 0.084 0.126 0.419

16 24 36 120

Minimum Threshold 12 12 12 12

0.628

180

12

a b

Actual size 16

Preferred (Lower Bound) Actual font size 7 20 20 Actual font size 10 20 20 20

Actual font size 15 Font 50 Font 75

Adequate (Upper Bound) 18 Actual font size 6 18 Actual font size 9 18 18 18

Actual font size 13.5 Font 45 Font 68

Adequate (Lower Bound) Actual font size 5.5 16 16 Actual font size 8


Actual font size 4

Actual font size 6


Smallest lowercase letter height. Font size is the distance from the highest ascender to the lowest descender of any character in the font set. Assumptions: Contrast ratio >7:1; luminance >35 cd/m2.


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29

2.1.1.5 Dynamic versus Static Acuity An individual’s dynamic visual acuity of moving targets is not strongly related to static acuity. Thresholds for dynamic acuity increase rapidly for rates of motion exceeding 60 inches of visual field per second and are considerably higher with shorter viewing times or longer target travel distance. Both dynamic and static acuity decrease with age (Burg, 1966). 2.1.1.6 Accommodation (Focusing Abilities) Guideline 2.2: Typical Visual Deficiencies Designers need to be aware of typical visual deficiencies of their users, including nearsightedness, farsightedness, astigmatism, and aging eyes. This would require making text and images larger and with higher contrast on labels, displays, and documentation. Accommodation is the adjustment of the lens of the eye to focus light rays properly on the receptor cells of the retina. Normal accommodation and common types of inadequate accommodation abilities are shown in Figure 2.4. The figure shows what causes the common visual impairments of nearsightedness (myopia) and farsightedness (hyperopia) and describes the effect of age on the ability to focus on near objects (presbyopia). Presbyopia is the inability of the eye to focus sharply on nearby objects, resulting from loss of elasticity of the crystalline lens with advancing age. The average age of onset is 40 years. Astigmatism, which also decreases the ability to focus, is a defect in which the unequal curvature of one or more refractive surfaces of the eye, usually the cornea, prevents light rays from focusing clearly at one point on the retina, resulting in blurred vision.

2.1.1.7 Visual Field Figure 2.5 shows the binocular (for both eyes) visual field measured in degrees. The normal line of site can be as small as 10 degrees from the horizontal plane as shown in the figure. Figure 2.6 shows the monocular visual field (for the right eye). The field for the left eye would be the reverse, or mirror image, of the right eye (Woodson and Conover, 1964). 2.1.1.8 Color Vision There are two types of receptor cells in the retina: rods and cones. The rods function under dim light and do not respond to color. They are located on the periphery of the retina away from the fovea, which is in the center of the retina, where vision is the sharpest. The cones Retina

Normal far vision – Light focused on retina

Lens normal Object

Normal near vision – Light focused on retina Lens curved

d

Age

20

40

60

d = nearest point of focus (average inches)

4.0

8.8

40.0

Nearsightedness (elongated eyeball and/or excess curvature of lens) – Light focused in front of retina for far objects Farsightedness (shortened eyeball and/or inability to increase lens curvature sufficiently as in aging) – Light focused in back of retina for near objects

FIGURE 2.4

Common visual impairments and focus point distances for different ages.


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30


50° 15°

Color vision limit 30°

94°

30°

e 70°

al lin Norm t h of sig 94°

0°

FIGURE 2.5

Normal visual fields in vertical and horizontal directions for both eyes.

function under relatively higher-intensity light and respond to color. They are located primarily in the area of the fovea. Factors that influence color vision are brightness (i.e., light intensity), hue (dominant wavelength), and saturation (pureness of color). As illumination decreases, so does color vision sensitivity. Not all zones of the retina are equally sensitive to color. Toward the periphery, objects can still be distinguished even though their color cannot. Some colors are recognized at greater angles away from the fovea than others. Figure 2.7 shows the limits of the retina zones in which the various colors can, under normal illumination, be correctly recognized (Woodson and Conover, 1964). Figure 2.8 shows the sensitivity of the human visual system to different colors (wavelengths of light). One curve shows the sensitivity of the cone receptors, and the second Nasal

Temporal 55° 30°

60°

Cornea Pupil opening

Eye

65°

34°

Occluded by nose

Occluded by eyebrow No color (under normal levels of illumination) Blind spot

70°

40°

Color area (cones) Occluded by cheek

Lens Retina

Fovea

(Sharpest vision at fovea with rapidly decreasing acuity toward retina periphery)

FIGURE 2.6 Monocular visual field of the right eye showing occlusions and color areas. (From Woodson, W. E. and Conover, D. W., Human Engineering Guide, University of California Press, Berkeley, CA, 1964. With permission.)


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31 Temporal Nasal White Blue Yellow Red Green

90° 65°

60°

50° 33° 25° 15° 10° Fovea

FIGURE 2.7

Retinal zones that are primarily sensitive to different colors.

curve shows the sensitivity of the rod receptors. The rod system takes over primarily after the eye is adapted to very low light levels, which is also known as dark adaptation (Glazer and Hammell, 1970). Guideline 2.3: Use of Color in Low Light Conditions Data from the human visual system have led to the following design recommendations: 1. Red and orange are poorly visible under low light conditions and should be avoided. 2. Blue, green, and yellow are equally visible under both low and higher light conditions and are good color choices, 3. Under low light conditions, blues and cyan colors are more visible. There is a shift in color sensitivity under these lower light conditions toward the blue end of the color spectrum. 48

100

Cone vision (color)

Rod vision (shades of gray)

Red

Yellow

Blue

20

Green

40

Orange

60

Violet

Relative visibility

80

0 400

450

500 550 Wavelength (mμ)

600

650

700

FIGURE 2.8 Relative visibility of different colors (wavelengths) for rod vision (dim light) and cone vision (brighter light). Curves are individually normalized and not normalized to each other.


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TABLE 2.4 Types of Deficient Color Vision and Their Frequencies Percentage of Population Affected Type

Effect

Trichromacy (three colors)

Normal color vision

Dichromacy (two colors) Protanopia (red blindness)

(Sees only two colors plus shades of gray) Inability to see hues other than yellow and blue with red and bluish-green seen as same shade of gray Inability to see hues other than yellow and blue with green and bluish-red seen as same shade of gray Inability to see hues other than red and green with yellow-green and purplish-blue seen as same shade of gray Inability to see hues other than red and green where color spectrum appears red at long wavelengths, green in the center, and red again at short wavelengths

Deuteranopia (green blindness) Tritanopia

Tetartanopia

Anomalous trichromacy

Protanomaly (red weak)

Deuteranomaly (green weak)

Tritanomaly Monochromatism

(Sees all colors but mismatches them, especially under dim light or small light sources) There is a foreshortened red end of the spectrum that requires more red than normal to match pure yellow; the spectrum is shifted, and normal yellow is seen as greenish There is no foreshortening of the red end of the spectrum, and more green is required to match pure yellow; the spectrum is shifted, and normal yellow is seen as orange More blue than normal is required to match cyan or blue-green Complete loss of color discrimination and poor acuity

Males 92.0

Females 98.0

1.0

0.02

1.1

0.01

0.0001

Very rare

Very rare

Very rare

1.0

0.02

4.9

0.38

Very rare 0.003

Very rare 0.002

2.1.1.9 Color Vision Deficiencies Approximately 8% of males and 2% of females have some degree of deficient color vision. Table 2.4 describes the various types of deficient color vision and their relative incidence in the U.S. population (Israelski, 1978). An example of the effects of color vision impairments is shown in Figure 2.9, which shows a patient monitor as seen by users with normal color vision, tritanopia, red blindness (protanopia), and green blindness (deuteranopia).


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33

Normal color vision

Tritanopia

Red blind (protanopia)

Green blind (deuteranopia)

FIGURE 2.9 (See color insert following page 564.) Comparison of colors seen by various users of a patient monitor with normal and deficient color vision, including tritanopia, protanopia, and deuteranopia.

2.1.1.10 Color Discrimination Recommendations Up to eight saturated surface colors (excluding black and white) can be used for color coding with practically error-free discrimination for color normal people. Color coding with more than eight colors produces higher error rates. Fewer colors would place less demand on memory. Under strictly controlled conditions, with a high level of training and the use of various combinations of hue and saturation, up to 50 colors can be identified with high accuracy. In any color-coding scheme, colors should subtend a visual angle of at least 15 minutes of arc. Guideline 2.4: Color Only One Form of Coding In any design, color coding should be a redundant information source and never stand as the only means of coding. If the population for which equipment is being designed is known to include significant numbers of color-deficient users, color coding should be avoided.

However, if a significant number of users may be color-deficient and color coding is still desired, only three colors may be safely used according to military aviation standards for the Army and Navy (Army-Navy Aeronautical Specification AN-C-56). These colors are not pure colors and therefore allow better discrimination for color-deficient users: • Aviation red (MIL-C-2505A Red) • Aviation green (MIL-C-2505A Green) • Aviation blue (MIL-C-2505A Blue) Other red, green, and blue hues may cause confusion. Colors are not recommended for use at larger distances because blue and green are more likely to be confused. White or yellow should not be added to the code because of probable red–yellow and green–white confusion for color-deficient individuals.


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TABLE 2.5 Recommended Printed Color Codes Using the Munsell Color System 8-Color Code n 1R 9R 1Y 7GY 9G 5B 1P 3RP

7-Color Code

6-Color Code

5-Color Code

4-Color Code

p

n

p

n

p

n

p

n

p

999 892 946 960 1099 1087 1135 1003

5R 3YR 5Y 1G 7BG 7PB 3RP

1008 890 1128 1103 1095 1133 1003

1R 3YR 9Y 5G 5B 9P

999 890 1131 1101 1087 1005

1R 7YR 7GY 1B 5P

999 884 960 1093 1007

1R 1Y 9G 1P

999 946 1099 1135

n, book notation of Munsell color system; p, Munsell production number; R, red; Y, yellow; G, green; B, blue; P, purple.

2.1.1.11 Recommendations for Printed Colors Guideline 2.5: Printed Color Recommendations For printed materials, colors should be chosen from the Munsell color systems (Table 2.5) (Cleland, 2004; Conover and Kraft, 1958; McCormick, 1970).

The Munsell color system is the system of color notation developed by A. H. Munsell in 1905 and identifies color in terms of three attributes—HUE, VALUE, and CHROMA— which are described symbolically. The HUE (H) notation of a color indicates its relation to a visually equally spaced scale of 100 hues. There are five principal and five intermediate positioned hue steps within this scale. The hue notation in general use is based on the 10 major hue names: Red (5R), Yellow-Red (5YR), Yellow (5Y), Green-Yellow (5GY), Green (5G), Blue-Green (5BG), Blue (5B), Purple-Blue (5PB), Purple (5P), and Red-Purple (5RP). The VALUE (V) notation indicates the lightness or darkness of a color in relation to a neutral gray scale, which extends from absolute black (value symbol 0/) to absolute white (value symbol 10/). The symbol 5/ is used for the middle gray and for all chromatic colors that appear halfway in value between absolute black and absolute white. The CHROMA (C) notation indicates the degree of divergence of a given hue from a neutral gray of the same value. The scale of chroma extends from /0 for a neutral gray to /10, /12, /14, or further, depending on the strength (saturation) of the sample to be evaluated. 2.1.1.12 Recommendations for Colored Lights Guideline 2.6: Recommendations for Colored Lights Colored lights need to be chosen using available data to have good recognizability and the least amount of confusion. Table 2.6 recommends colors and describes their effect on color recognition of small-point light sources near the threshold of visibility (Dreyfuss, 1966). Table 2.7 shows 10 colored light choices that were shown to reduce confusion error; that is, these 10 wavelengths had a less than 2% misidentification error in experimental studies


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TABLE 2.6 Recommendations for Colored Lights Color Discrimination a

Easiest Easier Difficult More difficult More difficult

Colors Recommended Red and green lights are easiest to recognize for color-normal individuals. White light is the next easiest to recognize. Yellow (or orange) is more difficult to recognize. Blue and green lights are very difficult to differentiate at a distance greater than10 feet. Yellow, white, and orange lights are difficult to differentiate at distances greater than 10 feet.

Source: Based on Dreyfuss, H., Measure of Man (2nd ed.), Watson Guptill, New York, NY, 1966. The best set of three colored lights is red, green, and white.

a

(Chapanis and Halsey, 1956). Use of the values in Table 2.7 increases the number of color choices that are easily recognizable and less likely to be misidentified.

2.1.1.13 Recommendations for Color Combinations (Legibility and Visibility) Guideline 2.7: Recommendations for Color Combinations Research has shown that the following lists are the best combinations of colors for providing good legibility, discrimination, and visibility.

The most legible color combinations for text are listed in order of legibility (Dreyfuss, 1966): 1. Black on white (most legible). 2. Black on yellow (most attention gained). 3. Green on white. TABLE 2.7 Ten Colored Light Choices That Have Less Than a 2% Misidentification Error Rate RGB Values Color Description Red Burnt orange Orange Yellow Yellow-green Green Light green Cyan Pale blue Medium blue

Wavelength (nm)

R

G

B

642 610 596 582 556 515 504 494 476 430

255 255 255 255 168 18 0 0 0 28

12 137 192 247 255 255 255 255 184 0

0 0 0 0 0 0 76 204 255 255

Source: Based on Chapanis, A. and Halsey, R., J Psychol, 42, 99, 1956. RGB, red, green, blue.


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4. Red on white. 5. White on blue. 6. Combinations of pure red and green or red and blue should not be used. 7. White on black may cause problems of smearing or irradiation of white on the black background if printing or electronic displays are not carefully controlled. For visibility of opaque colors under typical light conditions, the following colors and combinations are recommended: • • • •

Yellow is the most luminous and visible. Orange and red-orange hold maximum attention value. Blue is likely to be out of focus and indistinct. Red on blue or blue on red should not be used since each focuses differently on the retina and creates an induced three-dimensional effect called chromosteriopsis.

2.1.1.14 Dark Adaptation The rod system takes time to function efficiently (becoming dark adapted) after the eyes experience a change from bright to dark illumination conditions. There are both physiological and neurochemical changes that occur on dark adaptation. The pupils dilate, and the rod receptors become more sensitive. Guideline 2.8: Red Maintains Dark Adaptation The use of red goggles or red lighting does not affect rod vision and is useful in maintaining dark adaptation.

Figure 2.10 shows the lowering of visual thresholds as time in the dark progresses (Glazer and Hammel, 1970; Nutting, 1916). Full dark adaptation can take as long as 30 minutes, 5

Log of threshold brightness (μml)

Centrally fixated fields

Visual angle 2°

4

3° 3

5° 2

10° 20°

1 0

10 20 Minutes in the dark

30

FIGURE 2.10 Dark adaptation. Threshold of seeing decreases as a function of time in darkness and width of visual field or angle.


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37

B

A

FIGURE 2.11 Distance and perceived size of objects. Although arrows A and B subtend the same visual angle at the observer’s eye and therefore produce images of the same size on his retina, arrow A is seen to be farther away and hence appears to be larger.

but significant adaptation can take place in 8 to 10 minutes. The human eye also adapts to bright light chiefly by constricting the pupil. On subsequent exposure to low light levels, the pupils will normally dilate, after which the rod system starts the chemical process of increasing sensitivity to these low light levels.

2.1.2 VISUAL PERCEPTION There are known limits to human visual perception and its processing. Some of these visual limitations and resulting common errors are summarized in this section. Awareness of these common limitations will reduce the display of ambiguous information. Chapter 8, “Visual Displays,” provides practical recommendations for the design of visual displays. 2.1.2.1 Distance and Perceived Size The principle of perspective states that objects appear to be smaller when they are farther away. Distance and perceived size are related in the manner shown in Figure 2.11. We learn in childhood the distance–size relationship, and therefore it is automatically taken into account when we observe distant objects. If two objects that are actually the same size are perceived as being at different distances, the one that seems to be farther away will look larger. However, designers may unintentionally create misperceptions in size perception, as in Figure 2.12.

FIGURE 2.12 Perceived size of distant objects. The cone on the right is perceived as being larger than the identical cone at the left only because it seems to be farther away. (From Kaufman, L. and Rock, I., Scientific American, July 1972. With permission.)


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FIGURE 2.13 Object size misperception. The horizon moon appears to be farther away, although it is not. The viewer automatically takes the apparent distance into account. The viewer then unconsciously applies the rule that, of two objects forming images of equal size, the more distant must be the larger. (From Kaufman, L. and Rock, I., Scientific American, July 1972. With permission)

2.1.2.2 True Object Size The same concept applies to perceived object size in the presence of misperceived visual reference information. The apparent-distance theory holds that this is what happens in the case of the moon illusion and is illustrated in Figure 2.13. Guideline 2.9: Object Size and Distance Misperceptions Designers should not create objects that may be misperceived by users because of well-known visual processing limitations and illusions.

2.1.2.3 Common Visual Illusions The human visual system is easily fooled by visual illusions. Designers need to be aware of these visual illusions and of course avoid any graphic treatments that might trigger these problems of unreliable visual interpretations. Parallax error is one form of visual illusion that is commonly encountered and must be avoided. The error is seen as an apparent change in the position of an object, such as a medical device meter reading caused by a change of the observer’s line of sight, as illustrated in Figures 2.14 and 2.15.

Viewing from the left

Viewing from the right

FIGURE 2.14 Parallax errors. Note that the apparent reading of the meter needle position changes depending on the observer’s position from either the left or the right viewing angle. The image of the needle on the background display mirror is the true reading. Mirrors such as these are an aid to reduce parallax error.


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FIGURE 2.15 Straight-line illusions. Note how the observer sees lines that are not straight even when they are.

2.1.2.4 Perception of Motion Humans are often confused by slow relative motion between us and another object when there is no dependable frame of reference. Apparent motion can be induced, as is commonly observed in motion pictures (24 still frames per second with each shown twice) as well as television and cathode ray tube (CRT) displays (typically 50 or 60 frames per second). Another example of induced apparent motion is the phi-phenomenon, in which rapid successive flashes of individual lights arranged in a row or circle give the appearance of individual light-source motion. Guideline 2.10: Perception of Motion The designer should be aware of methods to create apparent motion in visual displays either to avoid confusion or to take advantage of them as potential information sources.

2.1.2.5 Flickering Lights Prolonged perception of flicker (over 20 minutes) in a light being flashed on and off causes visual fatigue and annoyance. A related concept is when flickering lights are perceived as being steady. The frequency at which a flashing light is perceived as having a continuous intensity level is called the critical fusion frequency (CFF). The CFF increases with increasing average light intensity and with decreasing proportion of the light–dark cycle occupied by the flash (percent modulation or duty cycle). CFF varies from 2 Hz up to 50 to 60 Hz for high-intensity light sources. 2.1.2.6 Photosensitive Epilepsy Flashing lights or the flicker of a computer monitor at certain speeds can trigger a seizure in susceptible individuals. This problem is called photosensitive epilepsy, photic epilepsy, or photogenic epilepsy (Harding and Jeavons, 1995): • Approximately 1 in 200 people have epilepsy, and of these, only 3% to 5% have seizures induced by flashing lights. • Photosensitivity is more common in children and adolescents and becomes less common after the early 20s.


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• The flicker frequencies that can trigger seizures vary greatly by individual and are in the range of 5 to 60 Hz. • Only 50% of photosensitive people are sensitive to 50 Hz, but 75% are sensitive to 25 Hz. Rates below 5 Hz are considered relatively safe. Guideline 2.11: Display Flicker Liquid crystal displays have no flicker and are preferable to CRTs in situations where flicker should be avoided, such as in medical display devices for use in the hospital rooms of seizure patients.

2.1.3 AUDITORY PERCEPTION This section describes the basics of human hearing abilities (see Chapter 10, “Alarm Design,” and Chapter 3, “Environment of Use,” for more detail and recommendations on environmental effects of sound). Loudness or sound volume is the subjective measure equivalent to sound intensity. The pitch or tone of a sound is the subjective measure equivalent to sound frequency. The loudness of a sound is perceived differently at various frequencies. For example, infusion pump alarm signals will be perceived as louder at higher frequencies. Thresholds for hearing sounds and feeling pain at different sound intensity levels are also a function of frequency (see Figure 2.16). 2.1.3.1 Loudness Measurements Two commonly used subjective loudness measures of sound intensity are phones and sones: • Loudness level (phones). Phones are the subjective measure of any tone intensity that is numerically equal to the sound pressure level (SPL) in decibels (dB) of a 140 Threshold of pain 120 Threshold of feeling

Prolonged exposure causes hearing loss

Intensity level (dB)

100

80

60

40 Words combine to make meaning 20

Threshold of hearing tones

Sounds become words Sounds can be heard

0

–20

FIGURE 2.16 Thresholds of hearing and pain. The curves show the intensity levels of sound as a function of frequency for two sets of thresholds: minimal threshold to hear pure tones and the threshold of pain. (From Woodson, W. E. and Conover, D. W., Human Engineering Guide, University of California Press, Berkeley, CA, 1964. With permission.)


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TABLE 2.8 Loudness for Common Noise Sources, Including Health Care Settings Noise Source Patient room at night (EPA recommended) Residential inside, quiet Patient room during day (EPA recommended) Household ventilating fan Automobile at 50 feet Typical patient room—peaks Anesthesia equipment—peaks ICU—peak sound levels IV pump alarm Hospital beeper Operating room—peaks Inside MRI machine Punch press, 3 feet Nail-making machine, 6 feet Pneumatic riveter

SPL (dB)

Loudness (sones)

35 42 45 56 68 70 76 80 85 89 90 95 103 111 128

570 >570 >1,800

Guideline 2.13: Improving Information Discrimination Information discrimination can be increased by allowing relative judgments to be made, presenting multidimensional stimuli, and increasing the rate of sequential presentation of stimulus material.

2.2.1.2 Attention People usually attend to only one source of sensory information at a time. This is true because we are basically single-channel processors, although we may do some time sharing of attention similar to the way a single-processor computer multiplexes or multitasks. Information from other unattended input channels is not totally blocked but rather is processed only incompletely with the result that sometimes certain kinds of information do get through. For example, individuals at a cocktail party can attend to a single voice amidst a babble of competing voices (the so-called cocktail party effect) yet still hear and recognize their own name being spoken in another conversation. A possible design implication is that a simple alternating tone or flashing light could precede important information presented in a channel likely to be otherwise unattended. 2.2.1.3 Vigilance (Sustained Attention) Psychologists are beginning to understand more about human performance during long periods of sustained attention (usually called vigilance activity) such as during a search for infrequent trouble signals on a display (e.g., missed heartbeats on a patient monitor). Table 2.14 lists various task conditions that affect human performance during prolonged vigilance (Van Cott and Kincaid, 1972; Wickens and Carswell, 1997).

2.2.2 SPEED OF INFORMATION PROCESSING 2.2.2.1 Reaction Time The time it takes for a person to react to an input stimulus and initiate a response is called reaction time. Simple reaction time (RT) involves only one response to a single stimulus.


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TABLE 2.14 Task Conditions That Affect Vigilance Performance Improved Probability of Signal Detection • Simultaneous presentation of signals to dual channels • Observers monitoring display in pairs; members of pairs permitted to speak with one another; 10 minutes of rest each 30 minutes of work; random schedule inspection by supervisor • Introduction of artificial signals during vigilance period to which a response is required • Introduction of knowledge of results of artificial signals Decreased Probability of Correct Detection • Introduction of artificial signals for which a response is not required • Higher or lower task load • Introduction of a secondary display monitoring task • Users report only signals of which they are sure Change in Probability of Detection with Time • A short pretest followed by infrequently appearing signals during vigilance • High initial probability of detection, decreasing rapidly • A few pretest signals before vigilance period • No CR reduces decrement in probability of detection with time • Prolonged continuous vigilance • No CR decreases probability of correct signal detection

Simple RTs for the different senses are provided in Table 2.15 (Brebner and Welford, 1980; Pierce and Karlin, 1957). Response time, a related concept, is the sum of reaction time, the cognitive processing time to an input stimulus, and the time to generate a response. The processing time is sometimes called think time. Choice RTs require a choice to be made among a number of stimuli and responses and typically are longer than simple RTs. The rate at which information is processed, such as during choice RT, is dependent on many factors (e.g., sensory channel, the kind and intensity of stimulus) (Pierce and Karlin, 1957). RT can decrease with training (up to 10%), the use of an alerting signal, an increase in intensity or duration of the stimulus, and optimized compatibility between delivered stimulus and expected response. RT increases (deteriorates) generally with age, fatigue, and the use of central nervous system– depressing drugs, such as alcohol. TABLE 2.15 Simple Reaction Times for Various Sensory Stimuli Stimulus Type Visual Auditory Tactual (haptic) Pain Cold Warm Movement (body rotation)


Reaction Time (ms) 150–225 120–185 115–190 400–1,000 150 180 520

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51 50

Errors per 100 moves

40

Fast speeds (8 decisions per minute)

30

20

Slow speeds (3 decisions per minute)

10

0

0

5 10 20 30 40 Load (number of columns displayed)

50

FIGURE 2.23 Speed versus accuracy trade-off (error rate versus speed of decisions). These data come from a study using a panel of varying numbers of columns of comparison numbers. The mental load increased with the number of columns displayed. The fast and slow speeds were, respectively, six and three decisions per minute. (From Mackworth, J. F. and Mackworth, N. H., J Opt Soc Am, 52, 713–716, 1958 and McCormick, E. J., Human Factors Engineering, McGraw Hill, New York, 1970. With permission.)

2.2.2.2 Speed versus Accuracy The so-called “speed-versus-accuracy trade-off” describes an approximately linear relationship with accuracy decreasing with either increased demands/faster response or additional workload. This relationship is shown in Figure 2.23 as the effects of increasing mental load on errors. 2.2.2.3 Human Memory Experimental cognitive psychologists refer to three basic types of human memory: sensory information memory, working short-term memory (STM), and long-term memory (LTM). Sensory memory has less bearing on device design and thus is not discussed further. Longterm memory is further categorized into two main areas: procedural (memory for processes and how to do things) and declarative (memory for facts and what to do). Table 2.16 summarizes the most important and distinguishing characteristics of working, declarative, and procedural memory (Kyllonen and Alluisi, 1987). 2.2.2.4 Working Memory This kind of STM transiently holds new information from the senses or other mental processes as might a computer data buffer. STM can be characterized by having fast access and retrieval time, limited capacity, and rapid loss of content unless actively attended to. The fast-access advantage of STM is countered by its limited capacity. The capacity is limited to five to nine “chunks” of information. This concept of working memory size limitation is also known as the “magic number seven plus or minus two” (Miller, 1956). The unit “chunk” is not defined precisely but can be considered a psychologically meaningful unit of information for material to be placed in STM. A large but limited number of bits of information may be contained in a chunk. This depends on the schemes used to recode bits


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TABLE 2.16 Main Characteristics of Memory Systems Characteristic Primary function

Working Memory (Short Term)

Long-Term Memory Declarative

Center of all thought/learning Stores meaning of inputs

Subset of declarative

Procedural Permanent storage of how-to knowledge

Storage of facts

Temporary storage in flux Capacity

Highly limited, 7 ± 2 chunks

Unlimited

Unlimited

Contents

Primarily acoustical codes

Semantic codes (primary)

Same as declarative

Secondarily visual/spatial

Spatial codes Acoustic codes Motor codes (physical movement skills) Temporal codes

Information units

Same as declarative

Concepts Schemata/frames/scripts

Production rules from very specific to general (if–then rules)

Organization

Same as declarative

Hierarchical with multiple levels of complexity

Flat

Learn/forget processes

Decays with time (73 seconds Learning by being told for one item, 7 seconds for (passive advice taking) three items)

Generalizations (inductive and deductive)

Increased decay time with rehearsal

Encoding

Learn by doing (active practice)

Interference from similar stimuli

Limited by retrieval paths and associations

Strengthening and reinforcement

Displacement (3–7 slots)

Very slow decay

Discrimination Analogies Problem solving

of input information into psychologically meaningful units. For example, if an individual had to memorize randomly generated alphabet letters, each letter would be a chunk. But if the task involved the memorization of randomly chosen words, then each word would be a chunk. A chunk of medical information could be patient name, room, and bed position. Information can be held in STM or working memory for as long as it is actively given attention, which usually involves rehearsing or reviewing the material over and over again. There is a human tendency to store information in working memory in an acoustical form whenever possible. The practical implication is that devices should be designed to minimize listening and talking when information is being held in working memory because these activities interfere more with STM.


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2.2.2.5 Long-Term Memory LTM has capacity ranging from 109 to 1015 or more bits of information. Retrieval from declarative LTM (facts) is slower than from working memory. Information gets transferred into LTM (either from STM or directly from the senses) only if appropriate links or associations can be established with psychologically meaningful material already in the long-term store LTM. Procedural memory (how to do things) is best learned by actively practicing a skill and appears to be very slow to decay (e.g., riding a bicycle, programming a computer, or remembering how to perform a laryngoscopy). This is because declarative LTM is organized much like a thesaurus with information having close meaningful associations being grouped near each other in the brain. People see and hear verbal messages more quickly and accurately if words with associated meanings are grouped close together, especially when competing signals are present. 2.2.2.6 Estimation and Decision-Making Abilities Humans are limited in both estimation and decision-making abilities. Among the highestlevel mathematical operations that people can perform “in their heads” are first-order integration and differentiation; these are performed crudely at best. Even simple arithmetic operations are performed poorly as soon as a person is stressed by demands for higher speed or accuracy. Our guessing behavior or probabilistic estimation skills show the following human tendencies (Kahneman and Tversky, 2000): • Overestimation of true probability for low-probability events and underestimation of true probability for high-probability events (especially overestimation of the probability of chains of unlikely events and underestimation of cumulative risks of events over a long period of time, such as the relationship between smoking and cancer). • Overestimation of true probability for events viewed as favorable and underestimation of true probability for events viewed as unfavorable. An example of overestimation is playing the lottery with enormously high odds. An example of underestimation is feeling threatened by a neurosurgery with an 80% success rate. • Unwillingness to believe constant probabilities for outcomes of successive independent events (also called the gambler’s fallacy, e.g., disbelief that p = .50 for a tail on the next toss after a fair coin has been tossed 10 times, each time coming up tails). • Human decision making is often not logical and, depending on circumstances or on how a problem is framed, we can be risk aversive or risk seeking. • Humans want to avoid false alarms when it comes to safety-related events. Signal detection theory (Swets, 1964) makes certain predictions about our decision-making criteria levels depending on the relative costs for false alarms versus the benefits of hits or true correct decisions. These trade-offs between decision criteria level and outcome consequences are dynamic and sometimes rapidly change. For example, when we are answering yes/no questions as part of a dangerous disease-screening questionnaire, we answer “yes” more frequently with more lenient criteria for decision making if the benefits of early valid detection of the disease are very positive and the therapy is simple, and we answer “yes” less frequently with stricter criteria if the costs of a false alarm are high (e.g., a very painful therapy for the disease). • Humans rationalize and rethink decisions in ways that are not always obvious or predictable. At the root of these problems is the concept of cognitive dissonance as


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studied by Festinger (1957). When faced with two options, we need to resolve any conflict that each choice presents in terms of differences in positive and negative consequences. Dissonance or conflict may exist both before and after we make decisions. Among common mechanisms for resolving dissonance are the following: • Rationalization—After making a highly conflicted choice, we reduce dissonance by altering our beliefs about something for which we formerly had strong convictions. For example, we choose to spend time participating in a long, boring memory experiment for very little reward or pay. We later rationalize our choice by saying that the experiment really was not that boring but was in fact very interesting. • Selective exposure or filtering—We begin to notice only positive attributes or features of our conflicted choice and ignore or filter any subsequent negative attributes that reveal themselves. For example, after being prescribed a certain drug, you begin to notice with increased frequency many other people taking it and also notice the same drug appearing in television ads. • Commitment—We increase our confidence about the quality of our conflicted choice and become steadfast in this belief. This behavioral tendency is related to sunk-cost bias, whereby we are not willing to give up or cut our losses on a bad investment that keeps falling in value. • Defensiveness—We resist any challenges to the soundness of our conflicted choices and categorize all feedback on our decision as wrong or biased. • Regret—We admit that we made the wrong conflicted decision and wish we had made a different choice. An example is the so-called buyer’s remorse we experience after making a large expenditure for an item of questionable quality (Malle, 2001). Regret has been shown to have different long-term and shortterm strengths. − Short-term regret is stronger for actions or commitments taken, such as buying a house. − Long-term regret is stronger for inactions, such as the trip you never took.

2.3 HUMAN RESPONSE CAPABILITIES In addition to limits in the ability to sense, perceive, and process information, humans have significant limitations in response capabilities after initial cognitive processing is finished. Only highlights of this broad topic are covered in this section, and the reader is referred to more in-depth treatments (see Eastman Kodak Company, 1989; Salvendy, 1997). Physical response data, such as strength, reach, and endurance, are covered in Chapter 4, “Anthropometry and Biomechanics.”

2.3.1 SPEED OF MOVEMENT Speed of hand and arm movements are dependent on a number of factors (Brown, 1949; Fitts, 1954). Hand movement time increases nonlinearly as a function of distance moved (Figure 2.24): • Maximum hand velocity for distances less than 3.2 feet is about 10 feet/sec. • Maximum hand velocity is about 20 feet/sec for longer distances.


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55 1.0 0.8 0.6

Time (sec)

0.4

Total time

Primary movement time Reaction time

0.2

Secondary movement time

0.1 0.08 2

4

6 8 10 20 Distance moved (cm)

40

FIGURE 2.24 Hand movement time as a function of distance moved. Movement time is for nonrepetitive movements of an object by the right hand from one position to another with complete visual feedback. Primary movement time is the time taken to make the major movement toward a target after the reaction time delay. Secondary movement time is the time taken to make small final adjustments while reaching the target.

Figure 2.25 shows the relationship of hand movement distance as a function of time. Fitts’s law describes the relationship between the speed of a control movement versus its difficulty (Fitts, 1954). The law can be used to predict a wide variety of user movements, including those involving surgical tools, computer mouse movements, and foot controls. Movement time increases proportionately with distance to a target and decreases with

Movement of right hand (inches)

16 15”

Pick up object

12

10”

8 5”

Return

4

0

0

0.20

0.40 0.60 Time (sec)

0.80

1.00

FIGURE 2.25 The times shown are for the movement of one hand from an initial position to some other position to pick up an object and to return to the original position. Reaction time delay is not included. The operation of a control panel switch involves this type of movement. The curves illustrate that long movements take proportionately less time in relation to length than short movements. (From Barnes, R. M., Motion and Time Study, John Wiley & Sons, New York, 1963 and Glazer, S. and Hammell, R., Physical Design of Electronic Systems, Prentice Hall, Englewood Cliffs, NJ, 1970. With permission.)


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larger target sizes. Fitts’s law implies that constant ratios of movement accuracy and movement distances result in constant movement times. It is beyond the scope of this chapter to describe the details. An excellent description of Fitts’s law and its practical applications for product design is found in Knight (1987).

2.3.2 PRINCIPLES OF MOTION ECONOMY Industrial engineers have completed large numbers of time and motion studies that suggest that the following principles may be used to increase the speed, accuracy, and ease of manual operations. Such tasks are not satisfying to most workers and may lead to errors as well as musculoskeletal disorders. See Chapter 16, “Hand Tool Design,” for more detailed guidance on design to accommodate human limitations in body movement and Chapter 12, “Workstations,” for guidance on the design of workspaces to include manual tasks. Guideline 2.14: Principles to Achieve Motion Economy The following principles, adapted from Barnes (1963), should be considered: 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16.

17. 18.

Avoid repetitive manual tasks. Normally, machines are better for repetitive tasks. Both hands should begin and complete their motion at the same instant. The hands should not be idle except during rest periods. Motions of the arms should be made simultaneously and in opposite and symmetrical directions. The motion sequence that uses the fewest steps is the best for performing a given task. Horizontal hand movements are faster than vertical. Hands should be relieved of all work that can be performed more advantageously by the feet or other parts of the body. Where possible, work should be held by jigs or vises so that hands are more free to operate. Tools, materials, and controls should be located in an arc around the workplace and as near the worker as possible. Tools and materials should be pre-positioned to eliminate searching and selecting. Two or more tools should be combined whenever possible. The height of the workplace and the chair should preferably be arranged so that alternate sitting and standing at work is easily possible. Continuous, curved motions are preferable to straight-line motions involving sudden and sharp changes in direction. Ballistic movements are faster, easier, and more accurate than restricted or controlled movements. Rhythm is essential to the smooth and automatic performance of an operation, and the work should be arranged to permit easy and natural rhythm whenever possible. Successive movements should be so interrelated that one movement passes easily into the next, each ending in a position favorable for the beginning of the next movement. A movement is less fatiguing if it occurs in the direction that takes the greatest possible advantage of gravity. When a forcible stroke is required, the movements and the material of the worker must be arranged so that when the stroke is delivered, it has reached its greatest momentum.


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19. Momentum should be reduced to a minimum if it must be overcome by muscular effort. 20. Hesitation or the temporary and often minute cessation of motion should be analyzed; its cause should be accounted for and, if possible, eliminated. 21. If a specific combination of movements has been determined as most suitable, emphasize form rather than accuracy even if this results in poor performance at the beginning of the learning period. 22. Arm movements that are mostly a pivoting of the elbow with small shoulder and upperarm action are faster and more accurate than those with a greater amount of shoulder and upper-arm action. 23. Manual limb movements terminated by mechanical devices take shorter periods of time compared to movements terminated solely by visual cues. 24. Single-hand visual positioning movements are faster and more accurate for short distances on a line 60 degrees from straight ahead on the same side of the body. 25. Two-handed visual positioning movements are most accurate straight ahead and fastest 30 degrees right or left of straight ahead. 26. In blind positioning movements, humans tend to undershoot long distances and overshoot short distances. Straight-ahead movements tend to be the most accurate. 27. Continuous movements in a horizontal plane are more accurate in certain angular directions from the midline of the body. For example, if 0 degrees is straight ahead, the most accurate movements for right-handed people would be 45 and 225 degrees and for lefthanded people 135 and 315 degrees. 28. Tremor or small vibrations of parts of the body degrade precision work and can be controlled by providing a visual reference, providing support of the body in general and any body part in particular, providing placement of the hand within 8 inches (20.3 cm) above or below the heart, or providing a limited amount of mechanical friction to absorb vibration energy.

2.3.3 SPEECH ATTRIBUTES 2.3.3.1 Loudness Levels of Speech This section describes some quantitative aspects of the human speech response and production capabilities. These attributes will become more important in device design as technology advances to allow more reliable and robust speech recognition systems. Table 2.17 shows average talker speech output in terms of sound pressure levels in dB (Morgan, 1963). Table 2.18 shows the distribution of talker loudness levels (Fletcher, 1953).

TABLE 2.17 Sound Pressure Levels of Speech 1 Meter from the Talker Normal Level (dB) Measure of Sound Pressure

Whisper (dB)

Minimum

Average

Maximum

Shout

Peak instantaneous pressure Speech peaks Long-time RMS pressures Speech minimum

70 58 46 30

79 67 55 39

89 79 65 49

99 87 75 59

110 98 86 70


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TABLE 2.18 Distributions of Speaking Volume for Persons Using the Telephone Volume Level Range (dB)a

Percentage of Speakers

75 a

Above sound pressure of 0.0002 millibars at a point 1 m from the talker’s lips.

2.3.3.2 Frequency Characteristics of Speech Because speech is a complex time varying quantity, its measurement is complex. Usually, speech is divided into a number of frequency bands and into a number of time segments (Figure 2.26). The average frequency is 128 Hz for males and 256 Hz for females. Most of the energy is below 1,000 Hz, with very little above 5 KHz (Morgan, 1963). Overall level (dB*) 90 Peak instantaneous 80 pressures

60

Speech-spectrum level (dB*)

50

A. Instantaneous pressure exceeded 1% of the B. RMS pressure time exceeded 10% of the time

40 30

Speech peaks 70 60

Long time RMS pressures Speech minima

20

50 C. RMS pressure exceeded 80% of the time

10 0 –10 100

200

*Re 0.0002 μ bar Male speech

500

1,000

2,000

5,000

10,000

Frequency (Hz)

FIGURE 2.26 The curves show how the intensity of male speech varies as a function of frequency and by different measurement criteria. At the upper right of the figure are corresponding overall levels for unanalyzed, unfiltered speech.


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2.4 HUMAN VERSUS MACHINE CAPABILITIES One way to summarize the basic elements of this introduction to basic human skills and abilities is to compare the relative advantages and disadvantages of humans versus machines or mechanical systems (Table 2.19). For example, we know that some human limitations when compared to machines include limited strength, low endurance, slower processing speed, less accuracy, emotionally impaired decision making, and severe STM and LTM limitations. On the other hand, machines do not fatigue, are faster, are more accurate, can more easily do parallel processing, and are much better at repetitive tasks. Many trade-offs

TABLE 2.19 Human versus Machine Capabilities Humans Limitations

Advantages

Force—Limited strength Endurance—Fatigues easily Speed—Significant time needed for decision making and movement time. Accuracy—Unreliable; makes constant and variable errors. Computing—Slow and error prone. Decision making—Best strategy not always adapted; emotions interfere. Information processing—Basically a single-channel processor, which is easily overloaded; performance greatly dependent on motivation. Limited short-term working memory; long-term memory, although large, has unreliable and slow access.

Visual acuity and range are very good. Visual information processing system extremely logical and flexible. Range of detection is extremely wide with good sensitivity for audition and vision. Perception—Ability to make order out of complex situations; detection possible under high noise. Can reason inductively; can follow up intuition. Very flexible; can easily change rules of operation with changes in situation. Attention is easily shifted; only essential information is selected for processing. When highly motivated can perform under adverse conditions with parts out of order (injuries).

Machines Limitations Decision making is limited. Inductive reasoning not possible. Must be monitored. All activities must be thoroughly planned and preprogrammed thoroughly. Must get careful maintenance. May not operate at all if some parts are broken.


Advantages Great forces are possible. Does not fatigue easily. High speed. Great accuracy attainable. Large short-term working memory. For narrow applications long-term memory is superior. Complex problems can be handled deductively. Excellent for repetitive work; unaffected by emotions and motivational needs. Can perform simultaneous operations easily.

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are required in assigning tasks to humans versus machines. The designer must fully understand the intended tasks, users, and use environment to be able to effectively select and design device functional attributes. The remainder of this book provides detailed guidelines on specific design topics.

RESOURCES Cobb, P. W. and Moss, F. K. (1928). The four variables in the visual threshold. Franklin Institute Journal, 205, 831. Hill, N. E. G. (1947). The recognition of colored light signals which are near the limit of visibility. Proceedings of the Physiological Society, 59, 560. Kahneman, D., Slovic, P., and Tversky, A. (1982). Judgment Under Uncertainty: Heuristics and Biases. Cambridge, MA: Cambridge University Press. Kling, J. W. and Riggs, L. A. (1971). Experimental Psychology. New York: Holt, Rinehart and Winston. McFarling, L. H. and Heimstra, N. W. (1975). Pacing, product complexity, and task perception in simulated inspection. Human Factors, 17(4), 361–67. Moray, N. (1972). Listening and Attention. Baltimore: Penguin. Stevens, S. S. (1947). Sensitivity to changes in the intensity of white noise and its relations to masking and loudness. Journal of the Acoustical Society of America, 19, 609. Stevens, S. S. (1951). Handbook of Experimental Psychology. New York: John Wiley & Sons. Stroh, C. M. (1971). Vigilance: The Problem of Sustained Attention. Oxford: Pergamon.

REFERENCES American Standards Association. (1954). The Relations of Hearing Loss to Noise Exposure. New York: American Standards Association. Barnes, R. M. (1960). Motion and Time Study. New York: John Wiley & Sons. Bevan, W., Maier, R. A., and Helson, H. (1963). The influence of context upon the estimation of number. American Journal of Psychology, 76, 464–69. Brebner, J. T. and Welford, A. T. (1980). Introduction: An historical background sketch. In A. T. Welford (Ed.), Reaction Times (pp. 1–23). New York: Academic Press. Brown, J. S. and Slater-Hammel, A. T. (1949). Discrete movements in the horizontal plane as a function of their length and direction. Journal of Experimental Psychology, 39, 84–95. Burg, A. (1966). Visual acuity as measured by static and dynamic tests: A comparative evaluation. Journal of Applied Psychology, 50, 460–66. Carlson, N. R. (2000). Foundations of Physiological Psychology (5th ed.). New York: Allyn & Bacon. Chapanis, A. and Halsey, R. (1956). Absolute judgments of spectrum colors. Journal of Psychology, 42, 99. Cleland, T. M. (2004). A Practical Description of the Munsell Color System and Suggestions for Its Use 1937. Whitefish, MT: Kessinger Publishing. Conover, D. W. and Kraft, C. L. (1958, October). The Use of Color in Coding Displays (USAF WADC-TR-55-471). Dayton, OH: Wright-Patterson Air Force Base. Cornsweet, T. N. (1970). Visual Perception. New York: Academic Press. Corso, J. F. (1967). The Experimental Psychology of Sensory Behavior. New York: Holt, Rinehart and Winston. Dreyfuss, H. (1966). Measure of Man (2nd ed.). New York: Watson Cuptill. Drury, C. G. (1975). Inspection of sheet materials—Model and data. Human Factors, 17(3), 257–65.


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Eastman Kodak Company. (1989). Ergonomic Design for People at Work: Vol. II. The Design of Jobs, Including Work Patterns, Hours of Work, Manual Materials Handling Tasks, Methods to Evaluate Job Demands, and the Physiological Basis of Work. New York: Van Nostrand Reinhold. Festinger, L. (1957). Theories of Cognitive Dissonance. Palo Alto, CA: Stanford University Press. Fitts, P. M. (1954). The information capacity of the human motor system in controlling the amplitude of movement. Journal of Experimental Psychology 47:381–91. Fletcher, H. and Munson, W. A. (1933). Loudness: Its definition, measurement, and calculation. Journal of the Acoustical Society of America, 5, 82–108. Foley, P. and Moray, N. (1987). Sensation, perception and systems design. In G. Salvendy (Ed.), Handbook of Human Factors. New York: John Wiley & Sons. Fraisse, P. (1963). The Psychology of Time. New York: Harper & Row. Geldard, F. A. (1972). The Human Senses. New York: John Wiley & Sons. Glazer, S. and Hammell, R. (1970). Man-machine interaction. Physical Design of Electronic Systems: Vol. 1. Englewood Cliffs, NJ: Prentice Hall. Harding G. F. and Jeavons, P. M. (1995). Photosensitive Epilepsy. Cambridge, UK: Mac Keith Press. Hatayama, T. and Tada, H. (1972). The experimental study of speed perception of the car on the road III: Speed judgements of drivers in the night time. Tohoku Psychologia Folia, 31(1), 4. Israelski, E. W. (1978). Commonplace human factors problems experienced by the colorblind: A pilot questionnaire survey. Proceedings of the 22nd Annual Meeting of the Human Factors Society, 347–51. Human Factors and Ergonomics Society, Santa Monica, CA. Kahneman, D. and Tversky, A. (Eds.) (2000). Choices, Values and Frames. New York: Cambridge University Press. Kaufman, L. and Rock, I. (1972). The moon illusion. Scientific American, July. Knight, J. L. (1987). Manual control and tracking. In G. Salvendy (Ed.), Handbook of Human Factors. New York: John Wiley & Sons. Kyllonen, P. C. and Alluisi, E. H. (1987). Learning and forgetting: Facts and skills. In G. Salvendy (Ed.), Handbook of Human Factors. New York: John Wiley & Sons. Lindsay, P., II. and Norman, D. A. (1973). Human Information Processing. New York: Academic Press. Mackworth, J. F. and Mackworth, N. H. (1958). Eye fixations recorded on changing visual scenes by television eye-marker. Journal of Optical Society of America, 52, 713–16. Malle B. F. (2001). Intentions and intentionality. In L. J. Moses and D. A. Baldwin (Eds.), Foundations of Social Cognition. Cambridge, MA: MIT Press. McCormick, E. J. (1970). Human Factors Engineering. New York: McGraw-Hill. Miller, G. A. (1956). The magic number seven, plus or minus two: Some limits on our capacity for processing information. Psychological Review, 63, 81–97. Miller, G. A. (1947). Sensitivity to changes in the intensity of white noise and its relations to masking and loudness. Journal of the Acoustical Society of America, 19, 609. Morgan, C. T. (Ed.) (1963). Human Engineering Guide to Equipment Design. New York: McGraw Hill. Norman, D. A. (1969). Memory and Attention. New York: John Wiley & Sons. Nutting, P. G. (1916). Effects of brightness and contrast in vision. Transcriptions of the Illuminating Engineering Society, 11, 939. Peterson, A. P. G. and Gross, E. E., Jr. (1967). Handbook of Noise Measurement. New Concord, MA: General Radio Corp. Pierce, J. R. and Karlin, J. E. (1957). Reading rates and the information rate of a human channel. Bell System Technical Journal, 36, 497–516. Richards, W. (1973). Factors Affecting Depth Perception: Final Report (Government Report AD-75926 1). Washington, DC: U.S. Government Printing Office. Riesz, R. R. (1928). Differential intensity sensitivity of the ear for pure tones. Physiological Review, 31, 867.


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Runeson, S. (1974). Constant velocity: Not perceived as such. Psychological Research, 37, 3–23. Salvendy, G. (1997). Handbook of Human Factors and Ergonomics. New York: John Wiley & Sons. Shower, E. G. and Biddulph, R. (1931). Differential pitch sensitivity of the ear. Journal of the Acoustical Society of America, 3, 275. Stevens, S. S. (1955). The Measurement of loudness. Journal of the Acoustical Society of America, 27, 815. Stevens, S. S. and Vollunan, J. (1940). The relation of pitch to frequency: A revised scale. American Journal of Psychology, 53, 329. Swets, J. A. (1964). Signal Detection and Recognition by Human Observers. New York: John Wiley & Sons. Van Cott, H. and Kinkaide, R. (1972). Human Engineering Guide to Equipment Design. Washington DC: U.S. Government Printing Office. Weinger, M. B. and Englund, C. E. (1990). Ergonomic and human-factors affecting anesthetic vigilance and monitoring performance in the operating-room environment. Anesthesiology, 73, 995–1021. Weintrab, D. J. and Virsu, V. (1972). Estimating the vertex of converging lines: Angle misperception. Perception and Psychophysics, 11, 277–83. Wickens, D. C. and Carswell, C. M. (1997). Information processing. In G. Salvendy (Ed.), Handbook of Human Factors and Ergonomics. New York: John Wiley & Sons. Woodson, W. E. and Conover D. W. (1964). Human Engineering Guide. Berkeley, CA: University of California Press.


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3 Environment of Use Pascale Carayon, PhD; Ben-Tzion Karsh, PhD; Carla J. Alvarado, PhD; Matthew B. Weinger, MD; Michael Wiklund, MS, CHFP CONTENTS 3.1 Space and Physical Constraints .................................................................................65 3.1.1 General Principles...........................................................................................65 3.1.2 Design Guidelines ...........................................................................................66 3.1.2.1 Device Form and Configuration .......................................................67 3.1.2.2 Security and Privacy .........................................................................69 3.1.2.3 Device Labeling................................................................................69 3.2 Lighting ....................................................................................................................70 3.2.1 General Principles...........................................................................................70 3.2.2 Design Guidelines ...........................................................................................71 3.2.2.1 Illumination ......................................................................................71 3.2.2.2 Special Medical Environments and Applications .............................73 3.3 Noise ..........................................................................................................................74 3.3.1 General Principles...........................................................................................74 3.3.2 Design Guidelines ...........................................................................................76 3.3.2.1 Critical Communications ..................................................................79 3.4 Climate: Thermal Environment .................................................................................80 3.4.1 General Principles...........................................................................................80 3.4.2 Design Guidelines ...........................................................................................82 3.5 Climate: Humidity .....................................................................................................82 3.5.1 General Principles...........................................................................................82 3.5.2 Design Guidelines ...........................................................................................83 3.6 Climate: Airflow and Pressure...................................................................................83 3.6.1 General Principles...........................................................................................83 3.6.2 Design Guidelines ...........................................................................................84 3.7 Vibration ....................................................................................................................85 3.7.1 General Principles...........................................................................................85 3.7.2 Design Guidelines ...........................................................................................86 3.8 Energy Sources ..........................................................................................................87 3.8.1 General Principles...........................................................................................88 3.8.2 Design Guidelines ...........................................................................................88

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3.9 Design for Infection Control ......................................................................................89 3.9.1 General Principles...........................................................................................89 3.9.2 Design Guidelines ...........................................................................................90 3.10 Case Studies ...............................................................................................................91 3.10.1 Cleaning of Flexible Gastroscopes .................................................................91 3.10.2 Patient Use Insulin Infusion Pump .................................................................92 Resources ...........................................................................................................................93 References ..........................................................................................................................93 This chapter describes the environmental issues that need to be considered in the process of designing effective and safe medical devices. It is important to recognize that the physical environment is only one element of the work system in which devices are used (Vincent, Taylor-Adams, and Stanhope, 1998; Weinger and Englund, 1990; Wiklund, 1995). According to Smith and Carayon (Carayon and Smith, 2000; Smith and Carayon-Sainfort, 1989), the work system is comprised of five elements: (1) the individual (end user), (2) tasks, (3) tools and technologies (including the medical devices), (4) physical environment, and (5) organizational conditions. In this chapter, we focus on the effects of the physical environment on end users and the interaction between the physical environment and a particular type of technology: medical devices. The ergonomic aspects of the environment include factors such as space and physical constraints, lighting, noise, climate, vibration, and electromagnetic radiation (Parsons, 2000). In this chapter, we examine each of these factors individually, addressing each environmental characteristic in the context of medical device use. People interact continuously with their environment in a dynamic manner, experiencing the environment as a whole (Parsons, 2000), and, therefore, typically do not respond to a single environmental factor in isolation. Moreover, various environmental factors can interact to produce synergistic or compound effects on human performance. Thus, medical device designers must consider the entire integrated physical environment. Both the main effects of environmental factors and their interactions can affect end users and their ability to use medical devices safely and effectively (Figure 3.1). Medical devices should be designed to be both easy to use and effective (i.e., do what they are supposed to do) in the context of expected use. This means that medical devices must be designed with consideration of the environment of use, the people who will use them, the other devices likely to be used at the same time, and the way in which the device will be used. Medical device designers must study the anticipated environments of use and test devices within them if they are to design devices that will succeed in those environments (Bruckart, Licina, and Quattlebaum, 1993; International Electrotechnical Commission [IEC], 2004; Wickens et al., 2004). This chapter covers medical devices and medical device use environments in the United States. Unusual or special environments, such as mobile (e.g., ambulance, helicopter; see Chapter 17, “Mobile Medical Devices”), public or outdoor (e.g., playgrounds, roadways), or home (see Chapter 17 and Chapter 18, “Home Health Care”) environments, require special design considerations. However, recommendations herein may be applicable to many health care environments and should be used as appropriate. This guidance is not intended to restrict medical device innovation or improvements in current design or use techniques, noting that device design is regulated and advised by many federal agencies (e.g., the Food and Drug Administration [FDA]) and national and international standards).


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FIGURE 3.1 The complex environment of hospital operating rooms (shown) or intensive care units provides substantial challenges for the medical device designer who must consider myriad factors, including lighting, temperature, noise levels, and the impact of other devices. (Photo courtesy of Frank Painter.)

3.1 SPACE AND PHYSICAL CONSTRAINTS The goal of human factors is to design systems that reduce human error, increase productivity, and enhance safety and comfort (Wickens et al., 2004). Designing ergonomic workspaces is one of the major ways to improve the fit between humans, medical devices, and the patient care physical space. Health care presents many challenging venues and physical design constraints for the device designer. Hospitals and clinics are forever adding patient care equipment, and on-site storage is often limited or not considered at the time of purchase. The designer needs to consider not only how the device will be used in the primary use environment but also how the device will interact with people and other devices in the available space and where the device will be stored when not in use. Additionally, devices designed for clinical arenas are often difficult to modify for mobile or home care, where use and storage space may be far more limited.

3.1.1 GENERAL PRINCIPLES The physical space in which a device will be used must be sufficiently large to accommodate the functions, people, and devices for which it is intended. In the case of health care, if the space is too constrained, not only could users be uncomfortable and device use impaired, but there might also be a greater risk of use errors resulting in harm to patients and caregivers. Devices that cannot be placed in a convenient location within a work area are likely to be used less effectively (and may even be used less often). Therefore, designing devices for use in a specific environment must consider the space available and the associated reach, line of sight, and related physical use requirements. Line of sight (i.e., ability to see a critical device attribute from a typical use position), access (i.e., ability to reach and manipulate the device), and clearance (i.e., space between use elements such as adjacent


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FIGURE 3.2 Helicopter interior showing substantial physical space limitations. (Photo courtesy of Wiklund Research and Design.)

controls) must consider the entire use environment. For example, if a device is likely to be placed atop an existing workstation (e.g., an anesthesia monitor placed on top of an anesthesia gas machine), then its design should accommodate the visual field of view and reach of both sitting and standing users. In some use environments, the presence of caregivers, the patient, and medical devices leaves little room for the addition of new devices without compromising clinical care. For example, in the transport care environment (e.g., ambulances and helicopters; see Figure 3.2), space is extremely limited, and clearance challenges can be substantial. Designers also need to consider typical task workflow and other environmental factors. For example, devices that require power cords or other connections may present a significant tripping hazard in a congested use environment occupied by fast-moving workers (e.g., a busy emergency room). Trip and fall hazards are relatively common in health care for both patients and workers. People could, for example, fall and sustain injuries by slipping on wet surfaces created by the device, tripping over cords or tubing associated with the device, or tripping over the device itself. If not disposable, the medical device may stay in the use environment for prolonged periods. Power supplies, device cleaning, and maintenance should be considered during design if the device is likely to reside in a patient’s room, for example, for many days.

3.1.2 DESIGN GUIDELINES The following specific design guidelines apply to physical and space considerations. Guideline 3.1: Device Visibility Medical devices should be visible in their expected use environments, which might range from outdoor locations to operating rooms. For example, a device commonly placed on top of a patient covered by medium-blue sheets should be a contrasting color to make the device easier to find. Similarly, devices frequently exposed to blood should not be colored red.


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Guideline 3.2: Adequate Space for Device Use The physical space in which a device will be used should be sufficiently large to accommodate the functions, people, and devices for which it is intended.

Guideline 3.3: Space Considerations in Device Design Device designers should consider the space available and the associated reach, line of sight, and related physical use requirements.

Guideline 3.4: Reach Requirements for Smallest Users Reach requirements of the device and environment should not exceed the reach of the smallest users as they operate a hand-operated device and/or activate a foot control (Wickens et al., 2004) (see Chapter 4, “Anthropometry and Biomechanics”).

Guideline 3.5: Line of Sight and Device Positioning Device displays should be readily seen and easily read by the users (see Chapter 8, “Visual Displays”) when placed in the expected range of locations and positions in the intended use environment. This requires proper positioning and a clear line of sight with respect to the device and other equipment in the area.

Guideline 3.6: Critical Controls Accessible Critical controls should be accessible and not placed in tandem with or too close to other critical controls or devices that may be activated/deactivated inadvertently (see Chapter 7, “Controls”).

Guideline 3.7: Device Clearances Device clearances should accommodate not only routine and nonroutine use but also cleaning and maintenance requirements. When designing clearances, the expected positioning of the device in the use environment and associated equipment and devices should be considered.

Guideline 3.8: Slipping and Tripping Hazards Designers should identify and minimize or eliminate slipping and tripping hazards associated with device use.

3.1.2.1 Device Form and Configuration A medical device’s form in a particular use environment can have a strong influence on its safety and usability. For example, an ultrasound scanner that works well in a hospital’s surgical unit might not be able to be on a helicopter ambulance because of its contextually large size and weight. In contrast, a new-generation handheld ultrasound scanner, which is light and compact, could easily fit in a helicopter. Similarly, medical devices that have a single, inflexible configuration may function well in some use environments but not in others, where configurable devices would have a distinct advantage. Guideline 3.9: Compactness for Use in Congested Areas Medical devices intended for use in congested care environments, such as intensive care units, should be as compact as possible to enable effective personnel movement and provide room for other essential equipment (see Chapter 12, “Workstations”).


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FIGURE 3.3 Anesthesia workstations may be used on either the patient’s right or left side to accommodate clinician preference, room configurations, and surgical procedures. (From Music Therapy World, mmmagazine and University of Michigan Anesthesia.)

Guideline 3.10: Self-Contained Devices Medical devices should be as self-contained as possible rather than spreading out unnecessarily in ways that interfere with effective personnel movement.

Guideline 3.11: Flexible Placement Within Use Environment Medical devices should enable various placements within the care environment. For example, anesthesia workstations should be equally usable when they are placed on the right or left side of a caregiver positioned in front of the patient’s head (see Figure 3.3).

Guideline 3.12: Cable, Tube, and Wire Management Medical devices should incorporate the means to organize cables, tubes, and wires so they do not become tangled or confused with one another or with those associated with other devices.

Guideline 3.13: Device Use In Small Workspaces When necessary, medical devices should facilitate use in small workspaces. Environments such as a helicopter ambulance might limit users to reaching controls placed on a front panel, precluding access to a back panel (see Figure 3.4). In such cases, placing a device’s power switch on the back panel would pose a major usability problem.

FIGURE 3.4 Ventilator mounting scheme (see right-hand wall) provides access only to the device’s front panel in this helicopter ambulance. (Photos courtesy of Wiklund Research and Design.)


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Guideline 3.14: Damage Resistance Medical devices used in rescue operations should be able to withstand frequent impacts and rough handling by personnel focused on a given emergency rather than handling equipment carefully. Therefore, a patient monitor used in an ambulance or during patient transfers might be equipped with an oversized handle (encouraging proper carrying) and shock-absorbing bumpers.

Guideline 3.15: Stable Platform to Prevent Tipping Medical devices should be sufficiently stable to withstand the likely types of impacts in the intended use environments. For example, a cart-mounted electrocardiograph should not be subject to tipping over when quickly pushed out of the way in an emergency or struck by a hospital bed being rolled down the hallway.

3.1.2.2 Security and Privacy Medical devices should be protected against accidental actuations such as might occur when bumping into a control panel. Also, some medical devices, such as infusion pumps used to deliver narcotic drugs, might be subject to unauthorized use or even malicious tampering (i.e., for drug diversion), suggesting the need for protective mechanisms. Finally, the details of medical care are not a matter of public disclosure. Federal laws strictly regulate the access and release of medical information. Guideline 3.16: Protection Against Inadvertent Actuation Controls on medical devices used in congested environments should be protected against inadvertent actuation by moving equipment and personnel that might bump against them (see Chapter 17, “Mobile Medical Devices”).

Guideline 3.17: Tamper Resistance Medical devices used in uncontrolled or unsupervised environments should be protected against tampering, including unauthorized use. For example, electronic controls might require the user to enter a password to unlock them or use a physical key to open a cover.

Guideline 3.18: Protecting Patient Privacy Where practicable, medical devices should be designed to prevent bystanders (e.g., hospital visitors and other patients) from viewing confidential patient information appearing on displays. For example, a computer display might be equipped with an overlay (e.g., polarizing film) that prevents viewing from the side. However, such overlays should not impede reading by intended users from likely viewing angles.

3.1.2.3 Device Labeling It is important for clinicians to rapidly locate, identify, and take note of any necessary precautions associated with a given medical device. Guideline 3.19: Indicate Prohibited Uses Medical devices should indicate pertinent, prohibited uses. For example, a medical device containing ferrous metals that is likely to be used near an MRI scanner should warn users about exposing it to an intense magnetic field.


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Guideline 3.20: Bold Labeling Medical devices should be labeled boldly to facilitate rapid identification, particularly when the device will be used or stored among with many other devices and supplies (see Chapter 13, “Signs, Symbols, and Markings”).

Guideline 3.21: Warning Label Placement Warning labels should be placed where intended users are most likely to see them. In some cases, multiple labels with the same warning may be necessary to ensure reliable viewing in various use environments and use positions.

Guideline 3.22: Consistent Color Coding Color coding should be consistent with local and national conventions. For example, controls on medical air valves used in the United States should be colored yellow, while those used in several European countries should be checkered black and white.

3.2 LIGHTING Characteristics of lighting in the use environment that are relevant to device design include illumination, luminance, contrast, glare, and shadow (see also Chapter 2, “Basic Human Abilities,” and Chapter 8, “Visual Displays”). Illumination is the amount of light falling onto a surface, commonly measured in units of lux, an SI unit (one foot-candle equals 10.76 lux). Luminance is the light generated by a surface and is commonly measured in units of candelas per square meter (cd/m2), an SI unit (one foot-lambert equals 3.426 cd/m2). The ratio of the amount of light reflected by a surface (luminance) to the amount of light striking the surface (illuminance) is called the reflectance. The formula for reflectance is Reflectance = π × luminance (cd/m2)/illuminance (lux) The contrast or luminance ratio is the ratio of luminance of any two surfaces or areas in the visual field. If there are large differences in luminance in the environment, the eye must adapt to these different levels as it moves from one area of the environment to another. Large differences in luminance in the environment can be sources of glare and cause visual discomfort, annoyance, or decreased visual performance (Sanders and McCormick, 1993). Glare is the reflectance of bright light off a surface that reduces visual contrast and impairs visibility. Glare can be direct if it is caused by a bright area in the visual field or indirect if it is caused by light being reflected by a surface in the visual field. Glare depends on the range of luminances in the visual field, on the reflectance of surfaces in the visual field, and on the position of the light source relative to the line of sight of the user. As the glare source becomes closer to the line of sight, discomfort increases (Sanders and McCormick, 1993).

3.2.1 GENERAL PRINCIPLES A poorly designed lighting environment will provide excessive or insufficient light for the intended tasks or activities. The effects of lighting on performance depend on the visual stimulus, individual characteristics (e.g., age), visual demands of the task, and physical environment (Boyce, 1997). Lighting parameters mainly affect the visual and perceptual


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TABLE 3.1 Recommended Illumination Levels by Location Location Bed, observation Bed, examination Operating table (directed locally) Operating room where other work is being performed X-ray processing room

Illumination (lux) 400 1,000 50,000–100,000 400–500 50

Source: From The Chartered Institution of Building Services Engineers, Lighting Guide—Hospitals and Health Care Buildings (LG2: 1989 ed.), The Chartered Institution of Building Services Engineers, London, 1989. With permission.

aspects of a task. For instance, light produced by medical devices and other equipment (e.g., bright lighting in an operating room) can create direct glare. Other lighting attributes affecting visual performance include illumination, luminance contrast (e.g., contrast between an object and its background), and reflectance. Display luminance can affect visual search performance when viewing radiographs (Krupinski, Roehrig, and Furukawa, 1999). Lighting levels vary widely depending on the health care use environment and the associated medical procedure or therapy. The visual requirements of clinical tasks performed with medical devices tend to be very high because of the precision required. For example, illumination levels of 10,000 to 20,000 lux are recommended for surgical procedures (Sanders and McCormick, 1993). Brighter illumination can improve the depth of field and therefore increase visual acuity. On the other hand, high illumination increases the likelihood of glare and shadows and may overburden the visual system. Even in the same use environment, lighting levels may vary considerably depending on the specific task being performed. For example, while operating rooms are normally brightly lit by general room lights as well as special spotlights (i.e., operating room lights), they might be dimly lit to facilitate a minimally invasive surgical procedure, such as arthroscopy, during which procedural guidance is derived from images on large displays. Therefore, devices used in the operating room, such as a ventilator, must be designed for use in variable lighting conditions. Lighting conditions will be even more variable for medical devices designed for patients to use in homes, public spaces, and outdoors. Recommendations for illumination levels in various areas of hospitals and health care buildings can be found for the United Kingdom in the Lighting Guide of The Chartered Institution of Building Services Engineers (1989). For North America, illumination recommendations can be found in the IES Lighting Handbook (Illuminating Engineering Society of North America, 1981) (see Tables 3.1 and 3.2).

3.2.2 DESIGN GUIDELINES 3.2.2.1 Illumination Designers should determine the visual demands of the tasks performed with the medical devices, such as the need for precision work and the characteristics of the visual environment. This information should be used to determine the level of illumination that


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TABLE 3.2 Recommended Illumination Levels by Visual Task Demands Visual Demands

Illumination (lux)

Visual demands are not high Performance of visual tasks—high-contrast items or large size Performance of visual tasks—medium-contrast items or small size Performance of visual tasks—low-contrast items or very small size Performance of exacting visual tasks (e.g., surgery)

100 300 500 1,000 3,000

Source: From S. N. Chengalur, S. J. Rodgers, and T. E. Bernard, Kodak’s Ergonomic Design for People at Work (2nd ed.), John Wiley & Sons, Hoboken, NJ, 2004. With permission.

should be provided for the tasks and thereby dictate the design of displays, controls, and device labeling. Guideline 3.23: Intended Use Environment Illumination The illumination levels necessary to safely and effectively use a device should be determined by knowledge of the intended use environments and user tasks.

Guideline 3.24: Adjustable Lighting Levels Light sources (e.g., a display backlight) should be adjustable (including the option to turn them off) to facilitate use in a variety of expected use environments.

Guideline 3.25: Luminance Ratios Visual performance is clearly affected not only by the type of display equipment (e.g., viewing box versus cathode ray tube monitor) but also by its luminance characteristics (Krupinski et al., 1999). The luminance ratios of device displays should be as follows: about 1 between tasks and adjacent darker surroundings, 0.33 between tasks and adjacent lighter surroundings, and 10 between tasks and more remote surfaces (Chengalur, Rodgers, and Bernard, 2004).

Guideline 3.26: Display Reflectance to Prevent Glare The reflectance of the device display should be such that it produces as little glare as possible. In general, surfaces should diffuse light (e.g., matte finishes) (Chengalur et al., 2004). If the medical device produces light, the light source should be shielded and not produce direct glare.

Guideline 3.27: Illuminated Displays and Controls Medical devices used in low lighting conditions (e.g., outdoors at night or in a diagnostic procedure room with its lights dimmed to facilitate minimally invasive procedures) should be illuminated as required to ensure proper device operation (see Figure 3.5). For example, keyboards and controls can be spotlighted, while displays can be backlighted.

Guideline 3.28: Attentuation of Transmitted Light If a device incorporates see-through materials (glass/Plexiglas between the user and what he or she is to view, then the design should consider the attenuation of transmitted light.


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FIGURE 3.5 Medic takes a soldier’s vital signs while traveling in a vibrating and dimly lit military transport aircraft. (Courtesy of Air Force Photo Gallery, image 081107-F-062E-477.) For instance, under normal lighting conditions, a reduction of 6% to 16% of the illumination was observed during the illumination’s passage through the walls or roof of infant incubators (Sjors et al., 1992). Incubators with double walls produce a greater decrease in illumination than incubators with single walls.

Guideline 3.29: Display Legibility in Bright Light As required, displays should remain legible when exposed to direct sunlight and intense artificial light sources (e.g., operating room lights).

Guideline 3.30: Display Cleanability Device displays should be designed so that they are easy to clean, thereby maintaining intended brightness (McCarthy and Brennan, 2003).

3.2.2.2 Special Medical Environments and Applications 3.2.2.2.1 Radiographic Imaging Environments Interpretation of medical/clinical images is dependent on image viewing conditions, including the lighting characteristics of the display equipment (e.g., viewing boxes and visual display). A range of guidelines has been developed that define appropriate radiological viewing conditions (see Table 3.3). 3.2.2.2.2 Endoscopic Devices The presence or absence of shadow can affect visual performance. The presence of shadows can provide depth cues that facilitate the reconstruction of three-dimensional images. Hanna, Cresswell, and Cuschieri (2002) demonstrated that the use of a shadow-inducing system facilitated mental processing of an endoscopic image reconstructed as a threedimensional picture of the operative field. On the other hand, the presence of shadows may affect visual performance as well as working postures (e.g., the surgeon has to adopt


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TABLE 3.3 Guidelines on Radiological Image Viewing Conditions Source World Health Organization (1982) CEC (1996) CEC (1997)

Brightness of Viewing Box (cd/m2)

Uniformity of Viewing Box (%)a

Illumination (lux)

1,500–3,000 2,000–4,000 >1,700

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