This research assessed the effects of inconsistencies in graphic direct manipulation interfaces. Objective and subjective measurement techniques were employed to determine how inconsistencies affected performance in an Apple Macintosh-based computer application called “The Personal Organizer.” Three groups of 11 participants, all familiar with the Macintosh computer, were given a set of similar tasks on different versions of the application in a pretest (control version), treatment (control or one of two inconsistent versions), post-test (control version) experimental design. Performance was measured using two objective measures: task completion time and the number of input control actions. Analysis of variance and correlational procedures were used to interpret these measures. A set of 29 bipolar semantic differentials were used to form a subjective measure of consistency. The linear sum of the scores on a subset of these items was used to create a composite measure of consistency. An analysis of variance procedure was performed on the composite measure, called the Preference Index. Results show that time and subjective measures are not identical in their ability to discriminate between inconsistent versions of the interface. It is concluded that the inconsistency of the interface has different effects on subject’s ability to complete tasks as compared to subject’s ability to rate interface consistency. Interface designers should be aware that subjective ratings of interface consistency need to be collected in concert with objective performance measures to assess the effects of graphical user interface inconsistencies upon human performance. / Master of Science