User-derived impact analysis provides a method for analyzing user behavior and presenting the results in a form that is useful for engineering groups. Since the user performance goal for VWS Version 2 was expressed in terms of performance on VWS Version 1, the first step was to measure V1 user performance. Table 2 shows their performance and evaluation scores. 20 6.6 0.3 Performance was measured using the work-speed performance score introduced by Whiteside et al. It is computed by multiplying the percentage of the task completed by the 3-minute constant and then dividing by the time taken on the task.
High scores represent fast performance.
The mean evaluation score of 1.7 was more favorable than the planned level of 0.25 for V2, indicating that no further work needed to be done in this area. Usability engineering requires that if a planned level of an attribute is demonstrably met, then further work on that attribute is not only unnecessary, but undesirable, because such work would involve taking resources away from product attributes whose planned levels were not yet met. These are shown as the left-hand column of Table 3. After identifying the problems, the videotapes were viewed again to estimate how much time was spent in each problem. Table 3 also shows these estimates for each subject.