Error and Injury in Computers and Medical Devices

Computers are playing an increasing role in everyday life, so it is not surprising that incidents involving computers have become a common matter of litigation. In a wide variety of technical, financial and other situations, people make decisions and perform responses based on the appearance of a computer screen. The increased reliance of computer controlled devices in medicine is particularly noteworthy because of the critical safety issues involved. As I will discuss below, this produces a conflict between the usual goal of interface design, intuitiveness and ease-of-use, and the safety of patients.   

When error occurs, the question naturally arises as to how one can apportion responsibility between the human and the computer: was the computer poorly designed or was the human negligent? 

In many cases, the critical component of the computer is its interface. Users/operators generally do not understand the computer's inner world of bits, bytes, files, ram, etc. Rather, they understand the computer through its interface, the text and images that appears on the screen. Hence, a popular saying in the computer world is that for the user: 

1. "The Interface Is The System"

User/operator actions can only be evaluated in the context of the interface. As an analogy, suppose a driver fails to see a STOP sign, runs an intersection and hits another car. Is it the driver's fault? If the STOP were highly visible, the answer would likely be "yes." If the STOP sign were hidden by foliage, then the answer is likely "no" because the information were beyond human ability to perceive and to respond. Similarly, the actions of a computer user can only be evaluated in reference to the quality interface design. 

In order to assess responsibility, it is important to understand how interfaces are designed. Below, I provide a brief outline of the methods which designers use in creating and evaluating computer and other man-machine interfaces.

Standard Practice in Interface Design

There are two principal issues which should be investigated when there is an accident involving computers. First, the computer interface should be evaluated for adequacy of design. Faulty design could be construed as negligence on the part of the designers. As discussed below, there are a number of standard criteria available for evaluating adequacy. Second, the interface design procedure should be closely examined. Just as there are standards for nursing practice and for medical care, there is standard practice for interface design. Although not formally codified, the standard practice is generally understood by interface professionals and is described in most design texts. Failure to follow the generally agreed practice is also a possible source of negligence.

Most interface design occurs in 4 overlapping phases. In addition, there is a 5th optional phase, which may or may not be performed.

2. Requirements gathering:

The designer studies users and their tasks and attempts to develop a set of requirements, which state what the interface should do. This stage is critical, since misunderstanding the users (or user classes if there are different groups) will guarantee a flawed interface design. 

3. Prototype development:

The next step is to develop a prototype with which to test users. In most cases, the initial prototypes are crude (sometimes even paper and pencil) and become more refined with user testing. (See next phase.)

4. Formative Usability Evaluation:

A good designer will continually test users in order to validate design and to test assumptions. Each design is really a guess until it has been tested and found adequate by a representative set of users (although there are other useful techniques - see below). The designer uses data obtained from each evaluation to refine and form (hence the name "formative") the design and prototype. The design is retested (looping the procedure back to phase 2), and the design further refined. In theory, testing stops when a predetermined set of benchmarks is reached and/or when the designer is simply satisfied. In reality, design typically ceases when time or money run out. 

5. Conversion of Prototype to Final Software:

Software engineers (programmers with little or no interface design experience in most cases) convert the prototype to a final form, usually in a faster and more efficient programming language. It is not uncommon for the design to change significantly in this process. Sometimes the software engineer makes what he/she considers small changes (which may, in fact, be major changes) in order to make programming easier and sometimes the efficient programming language simply cannot produce the design exactly. 

Summative Usability Testing:

Usability testers evaluate the final interface functionality. This may occur in-house, or the testers may go to customers in the field. Results are then used to improve the next software revision.

The scheme outlined here, with some variation, is standard practice by interface design and usability experts. However, this is not how all interfaces are designed. In many companies, people with little or no design, usability or human factors experience often create the interface. Some companies are run by techies who simply slap untested interfaces together as an afterthought, with software engineers performing interface "design." Graphic artists often design interfaces, especially on web sites, although they have no psychology or human factors training. Lastly, usability testing is expensive, so companies may skimp on this part of the project budget. Although the importance of human factors is becoming better understood in the technology world, inadequate design procedures are still common. 

A properly conducted design procedure generates a series of standard documents. The most important are the "requirements document," "design specification" and the "change control" documents. The requirements document says what the interface ought to do. The design specification document describes the interface in detail. It is usually written by the designer(s) so that the people programming the production code know what the design should be. There is often a deviation between the design and final product for the reasons already described. Lastly, the change control document is a formal notation of all changes in the design, usually after the design specification is written. There are almost invariably some design changes made up until the last possible second. Usually, there are specific people who must "sign off" on each document.

Standard Evaluation of Interfaces

Usability testing of the prototype is a critical part of design. There are many techniques for testing, but they fall into two general classes:

  • Those that that involve actual users

  • Those that do not involve actual users.

The first method, which has many variations, has users sitting at a computer and working with a prototype. At early development stages, the user may be merely exploring the software - viewing screen, clicking buttons, entering numbers, etc. With more refined prototypes, the user may be solving a simulated task which resembles his/her real work. The tester may simply write qualitative observations or may record quantitative data, such as number of errors and time required for task completion, and perform statistical analysis. Videotaping of users is common.

The second method is sometimes used to supplement, and occasionally replace the first if users are hard to obtain. This occurs if the users' time is very valuable (physicians, lawyers, or very highly skilled technical employees, etc) or if the users are geographically remote. Moreover, it is sometimes difficult to know who the users will be, so it may be misleading to test with any particular group. 

A common nonuser method is called "Heuristic Evaluation." A group of 3-5 usability experts and/or nonexperts judges the interface based on a set of specific criteria. Here are some criteria, which would be used to judge most interfaces: 

  • Simplicity: make the interface easy to use:

  • Design For Error: assume that the user will make errors. Make errors easy to reverse and/or find a way to prevent them, e. g., ask for confirmation on important actions. 

  • Make System State Visible: the user should know what is happening inside the computer from looking at the interface. 

  • Speak the user's language: use concepts with which the user is familiar. If there are different classes of user (say, novices and experts in the work), then be sure that both groups understand the interface.

  • Minimize Human Memory Load: human memory is fallible and people are likely to make errors if they must remember information. Where possible have the critical information on the screen. Recognition and selection from a list are easier than memory recall.

  • Provide feedback to the user: when the user makes an action, provide feedback that something happened. At the most basic level, the feedback may simply be a beep to indicate that a button press was recorded. At a higher level, the feedback may be a message that describes the consequences off the action in detail. 

  • Provide good error messages: When errors occur, provide the user with good information about the problem. Poor error messages can be disastrous, as in the Therac-25 case.

  • Be Consistent: Similar actions should produce similar results and objects, which are the same visually (colors, shapes), should be related in an important way. Conversely different objects should be indicated by different visual appearance. 

These same criteria can be used to judge the responsibility of the interface design in an accident. From experience, I'd guess the most common problems are lack of consistency, hiding of the system state and failure to design for error. (Most interface designers have never head of a Failure Mode and Effects Analysis.) 

Ease of Use vs. Safety: An Example of Medical Device Error

One problem in evaluating interface design is that safety and ease-of-use sometimes conflict. Interface designers are taught to make the interface "user friendly" and intuitive. Being intuitive, however, is a two-edged sword. The good news is that users learn the interface quickly and make fewer errors. In addition, an intuitive interface is more likely to be properly operated when the user is under stress, the time when people unconsciously fall back their innate and highly learned behavior. 

The bad news is that very notion of "intuitive" means that the user/operator won't have to think too much. In safety critical situations, this is not always desirable. People have a tendency to minimize their workload by using more and more general cues. For example, instead of reading a red warning label, they may learn to simply respond when they see the red text - it is much easier and faster to recognize color than to read text. If there is an unusual or unexpected message in red, the user will not notice the change because the cue is color, not the actual text. Similarly, users learn to make their responses "automatic" when they occur with great frequency. 

The classic example of ease-safety conflict is the Therac-25, which was a computer-controlled device for delivering measured bursts of radiation to cancer patients. Several patients being treated with the machine accidentally received fatal doses of radiation.

There were many problems with the Therac-25 (including poor error messages which failed to make the machine state visible), but I'll just comment on one aspect of the interface design. In the original version of the machine, the operator had to enter control parameters twice. First, they were typed into the computer and sent with by hitting the "enter" key. Second, the user entered the values into a control panel. This provided redundancy for a critical task. It seemed less likely that the operator would enter the same wrong values twice. Moreover, the computer could check to make sure that the values were the same.

From an ease-of-use standpoint, this was a clumsy design. The interface designers decided make the users life easier by removing the need to confirm values with the control board. As before, the user typed numbers and then hit the enter key to send the values. Instead of going to the control board, the values appeared again on the screen, and the user could confirm them by hitting the "enter" key a second time. This second confirmation was a replacement for the control board data entry. It was much a faster and more efficient interface design. 

Users soon began entering the values and then simply hitting the "enter" key twice without looking at the screen. The new system was easier, but the redundant check on the values was gone.

This was highly predictable because of phenomena called "automaticity" and "response chaining." When a person repeatedly performs a task requiring a standard and unvarying series of responses, then the responses chain together and effectively become a single response. Once started, the chain of responses runs off automatically. For example, a pianist learning a new piece might have to think about every note before hitting the key. After practice, the pianist simply runs off the series of responses without thinking. This reliance on "muscle memory" is obviously much easier, but thinking is removed from the task. The Therac-25 case was a particular bad example because the enter and confirmation responses were identical, which facilitates response linking. 


Since "The Interface is the system", an attorney investigating any accident involving a computer should examine the interface design. The first questions that should always be asked about an accident involving computers are:

  • Did the interface design meet the requirements of the task?

  • Did the interface meet standard evaluation criteria?

  • Was standard practice followed in the interface design? 

  • Was testing adequate? 

  • Were the test users appropriate? 

  • Were the usability test results properly interpreted and incorporated in the design? 

  • Were there "Change Control" and other formal documents on the design procedure?

  • Who designed the interface and what were his/her credentials?

  • If a safety critical situation, what was the tradeoff between ease-of-use and safety? 

There are also secondary issues which should also be examined, such as "Was user training adequate?", "If the user/operator could customize the interface, did he/she reduce interface quality?," etc. 

In this article, I have outlined the issues involved in determining responsibility in accidents involving computers. Computers and similar devices are truly "man-machine systems," where both components must function properly to avoid error. It is perhaps natural to examine only the user/operator actions, since they represent the visible "sharp end" of the system. In many cases, however, the major fault lies with the machine and/or the process used to develop the most important part, the user interface.

Copyright © 2003 Marc Green, Ph.D., All rights reserved. No portion of this article may be reproduced without the express written permission of the copyright holder. If you use a quotation, excerpt or paraphrase of this article, except as otherwise authorized in writing by the author of the article you must cite this article as a source for your work and include a link back to the original article from any online materials that incorporate or are derived from the content of this article.

This article was last reviewed or amended on Jun 15, 2017.