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Designing systems that are tolerant of human error becomes crucial when any task has potentially dangerous and costly consequences, or when the outcome is not easily reversible.

Financial and medical web applications are prime examples of sites where reducing human error is especially important. A central theme in designing for human error is to build a multilayered defense.

Designing for human error effectively requires addressing several aspects of error management, including the following.

Prevention: Eliminate the potential for error to occur by changing key features of the task or interface. This is always the preferred method for error management and also the most effective.

Reduction: Reduce the likelihood that the user will get into an error state when prevention is not possible by ensuring the user is aware of action consequences and by training users on both normal and error-recovery procedures.

Detection and identification: Ensure that if the user does err, the system makes it easy for the user to detect and identify the error.

Recovery: Following error detection and identification, ensure that the system facilitates rapid correction, task resumption, and movement to a stable system state.

Mitigation: Minimize the damage or consequences of errors if they cannot be recovered from.

Even when all other error management steps have been taken, errors will still be made, so systems should be designed such that catastrophic outcomes from human error are not possible.

Example: Error Recovery
The issues for error recovery are detection, identification, correction, and resumption. For the user the questions are simple: What happened? What do I do now?

Electronic commerce is a prime domain for error-recovery design flaws that can adversely affect usability. Consider, for example, the order form in (slightly modified from www.netopia.com).

Here, the form indicates that certain asterisked fields are required. The credit card selection menu is not indicated as a required field and is easy to miss (the card type is not really necessary and can be derived from the card number).

Also note the instructions at the bottom of the form telling the user not to use the Stop or Back buttons. These instructions help prevent users from accidentally performing multiple transactions or from mistakenly believing that a transaction was canceled when it was not.

The problem is that this error management measure hinders recovery from other error types. If the user forgets to select the credit card type from the pull-down menu, the commerce server indicates an error by displaying the screen shown in.

The user detects that an error has occurred from the message heading. The system even identifies the error as an invalid entry in “WXK_PAYMETH” field.

Unfortunately, that field name is only clear to the system and its programmers. Most users would not know from this that the problem is an incorrect entry for the payment method.

In fact, the credit card menu is not strictly a field and does not really reflect an entry as the message suggests.

This would likely cause more confusion. Even if the error is deduced by the user, correction is hundred by the message on the original order form that said, “Do not use the STOP or BACK button.” There is no other navigation on the error page, so using the Back button is, in fact, the only option.

A better way to handle incorrect data entries is to return the user to a facsimile of the order form, with the missing or erroneous data fields highlighted in red (or indicated in some other salient way).

Examples of correct entries can also be helpful. Minimally, error pages should identify errors in a way that users can understand and provide clear recovery paths when possible.
Simple, practical techniques, applied consistently, can dramatically improve the web site development process. More significantly, these fundamental task analysis methodologies please the ultimate critic: the user.
 

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