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.

Conclusion

Design of medical device interfaces and interactions differs significantly from design for non-medical devices. Error may produce more severe consequences, not to mention expensive litigation. Interaction design requires a somewhat different set of rules and "ease of use" cannot be the only concern.

About Human Factors MD

Based just outside of Toronto, Canada, Human Factors MD provides product design and evaluation services to medical software and device companies, and risk management services to healthcare providers. For more information, contact media@humanfactorsmd.com, or visit www.humanfactorsmd.com.

Human Factors MD
Making medical technology usable.™

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