Abstract
Usability evaluators used an 18-item, post-study questionnaire in three related usability tests. I conducted an exploratory factor analysis to investigate statistical justification to combine items into subscales. The factor analysis indicated that three factors accounted for 87 percent of the total variance. Coefficient alpha analyses showed that the reliability of the overall summative scale was .97, and ranged from .91 to .96 for the three subscales. In the sensitivity analyses, the overall scale and all three subscales detected significant differences among the user groups; and one subscale indicated a significant system effect. Correlation analyses support the validity of the scales. The overall scale correlated highly with the sum of the After-Scenario Questionnaire ratings that participants gave after each scenario. The overall scale also correlated moderately with the percentage of successful scenario completion. These results are consistent with the hypothesis that these alternative measurements tap into a common underlying construct. This construct is probably usability, based on the content of the questionnaire items and the measurement context.
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