A model of the interface design process is proposed that makes use of two interdependent levels of cognitive analysis: the study of the criterion task through an analysis of expert/novice differences and the evaluation of the working user interface design through the application of a practical interface analysis methodology (GOMS model). This dual analysis is reviewed in the context of HYDRIVE, a video-based intelligent tutoring system designed to facilitate the development of troubleshooting skills for technicians working on aircraft hydraulics systems. The initial cognitive task analysis enabled the identification of critical troubleshooting skills and troubleshooting procedures. It is found that, even with an in-depth initial cognitive task, the GOMS interface analysis resulted in significant and beneficial design changes. Two figures illustrate the discussion. (SLD)