Commonality analysis was used as part of an evaluation of the Preparation through Responsive Educational Programs (PREP), a project based on behavior modification for disruptive, poorly achieving, and potentially delinquent urban junior high school students. The objectives of this study were to understand how the program affected certain academic and non-academic outcomes, and to assess the practicability of commonality analysis. Data were collected over a 3 year period, from 221 youth, 112 of whom participated in PREP and 109 of whom were controls. The major questions in this research have been how much environmental, as opposed to personal, variables contributed to scores on achievement and other school-related behaviors, and the extent to which the PREP program might have affected these scores and behaviors. The results support PREP's techniques for improving English and mathematics classroom performance and argue for a greater emphasis on the social skills aspects of the urban program's curriculum. The use of commonality analysis in evaluating programs such as PREP's urban effort could provide a ready and clear description of where program efforts should be enhanced or changed. (PN)