In terms of task completion time, programming tasks were completed more quickly with the new interface than with the old by 11 out of the 12 nurses. The average time to complete programming tasks with the new interface was 18% faster (Figure 3), a difference that was found to be statistically significant (F(1,10)=12.17, p=0.0058).

Analysis of workload ratings showed that there was a 14% decrease in workload with the new interface over the old. This reduction in mental workload, however, was not found to be statistically significant (F(1,10)=1.06, p=0.33). Also, there was an approximately equal proportion of subjects rating a lower workload with the new interface as with the old (X2(1)=3.33, P>0.05 one-tailed). The only significant effects on workload came from an interface x task x repetition interaction (F(2,20)=8.62, p=0.002). The resulting effect of this interaction was a significant decrease in the workload reported for the new interface over 2 repetitions for 2 of the 3 programming tasks (Continuous and PCA+Continuous tasks), whereas the decrease was less substantial for the old interface (see Figure 4) .

In the interviews following the experiment, a majority of subjects who expressed a preference, preferred the new interface over the old (X2(l)=6~4, p<0.012 one-tailed). Nine nurses stated that they favored the new interface, 1 preferred the old, and 2 were neutral.

DISCUSSION
The results of this empirical evaluation replicate most of the results we had previously obtained with student nurses, confirming the hypothesis of this study. Measurements of nurses' performance with programming tasks indicate an overall improvement in performance with the redesigned interface compared to the existing Abbott interface.

The redesigned interface led to a 55% reduction in the total number of programming errors. Furthermore, there were no errors in sett
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Contact: Lois Smith
hfes@compuserve.com
310-394-1811
Human Factors and Ergonomics Society
1-Feb-1999