The paper describes a microcomputer disk, Data Based Program Modification (DBPM), which allows special and regular educators to monitor the progress of each student, thereby perceiving the effectiveness of the instructional program. The advantages of such a formative evaluation approach are considered. The development of the DBPM is outlined in its three stages: (1) identification of the behaviors to be measured in reading, spelling, and written expression; (2) development of technically adequate measurement procedures for measuring those behaviors; and (3) an exploration of alternative approaches for using the data to make decisions about instructional effectiveness. Results of pilot studies and field tests are reported, technical information is given, and additional services of the founding company (including training on procedures for setting goals and objectives and the application of the data for program evaluation) are described. (CL)