The National Council of Teachers of Mathematics (1991) has identified the use of computers as a necessary teaching tool for enhancing mathematical discourse in schools. One possible vehicle of technological change in mathematics classrooms is the Intelligent Tutoring System (ITS), an artificially intelligent computer-based tutor. This paper reports on 3 construct validity studies that have been conducted with 97 high school students in order to demonstrate the correspondence, or lack thereof, between the theoretical constructs of the Diagram Configuration (DC) Model of geometric proof-writing expertise (Koedinger & Anderson, 1990) and the hints and errors being recorded by an instantiation of the DC Model called ANGLE, an intelligent geometric proof tutor. Results of the studies supported the appropriateness of construct validity techniques for analyzing ITS data. The results partially confirm a hypothesized factor structure for the data. The paper concludes with a discussion of the results, including suggestions for modifications of the ANGLE program. (Contains nine references and six tables.) (Author/SLD)