A situation in which computer-assisted instruction was combined with teacher-taught classes and computerized diagnostic adaptive testing was examined. The computerized diagnostic adaptive test for pre-algebra signed-number lessons was programmed along with a computer-managed routing system by which examinees were sent to the instructional unit corresponding to the skill level at which they stopped in the initial test. Upon completing the course a computerized conventional post test was administered. It was found that results differed markedly from results obtained in a previous study. A cluster analysis was performed on the response patterns of the skills, and four different groups were found. A discriminant analysis indicated significant differences among the four groups in response patterns of the skills in signed number operations. It was determined that differences between prior and current instructional methods confused students and caused confusion in the post-test data, and that the scoring procedure of the adaptive testing was not sensitive to individual differences in information processing skills which were affected by the instructional method used in previous teaching. It was concluded that reliance should not be placed solely on test results determined by performance scores on a diagnostic pretest. (Author/CTM)