Cost-Justifying Usability, 2nd Edition

on this metric unless it is assumed that the situational factors between trials are

identical.

Going back to probability of task success, in practice, a trial is either suc-

cessful or unsuccessful for one of the three following reasons:

1. The user gives up before the time limit is passed.

2. The time limit for the trial is passed, and the user is asked to stop.

3. The user finishes within the time limit but does not complete the task to the

criterion (whether or not the user realizes that he or she has not completed

it).

In each case, it may be possible to engage in assessment of how close the user’s

result is to the criterion result. This is often not ...