The measurement of software usability has become an important issue in recent years. Metrics of usability include time, errors, questionnaires, ratings, and results of verbal protocols. Concurrent verbal protocol, a method in which the user "thinks aloud" while completing given tasks, has been heavily employed by software usability researchers who want to know the reason a user is having difficulties. Possible problems associated with using concurrent verbal protocol are (1) that verbalization may interfere with the processing required to complete the task, and (2) that subjects may not be able to monitor and express the information of interest to the researcher. A relatively new approach which may avoid these problems is heavily cued retrospective verbal protocol in which the user is presented subsequently with a representation (a video tape, for example) which helps him recall his thoughts during the task without interfering with task completion. This research compared the performance of subjects while completing tasks using both methods of verbal protocol. The verbal data collected by the two protocol techniques was compared to assess any information differences due to the methods of collection. No performance differences were found between the two protocol methods. Reasons for this lack of degradation due to concurrent verbalization are discussed. The kinds of information gathered were quite different for the two methods, with concurrent protocol subjects giving procedural information and retrospective protocol subjects giving explanations and design statements. Implications for usability testing are discussed. The two methods of protocol were employed in a comparison of two different size monitors, a 30.48 cm diagonal and a 53.34 cm diagonal. The subjects' performance, as measured by steps to completion, task completion time, and errors committed, was compared across the two monitors. Subjects were required to complete 12 tasks which varied in the difficulty of the windowing required. Subjective data were also collected in the form of task difficulty ratings, as well as a global measure of user satisfaction. These performance measures and subjective measures were compared across protocol methods as well as monitors. Performance data, as well as subjective data, indicate that on tasks that do not require extensive windowing, there are no difference between the two monitor sizes. As windowing difficulty increases, however, the large monitor's advantages become apparent. Tasks with a high level of windowing difficulty are judged to be easier and require fewer steps on the large monitor than on the small monitor. / Ph. D.