This report was written to provide guidance in the development and evaluation of alternative instructional approaches that hold promise of improving instructional effectiveness. The main focus of the report is on how to identify and test interactive relationships between individual differences among learners and instructional conditions or treatments. The report directs considerable attention to problems of measurement that are basic to diagnosis and evaluation. The report rests on the conviction that no acceptable substitute for careful empirical experimentation exists; an approach must be tried, often several times, before evidence sufficient for credible evaluation is available. Underlying this conviction is commitment to principles of measurement. Detecting and quantifying the effects of instruction require dependable measurement. Thus, a portion of this report concerns (1) the development of measures of learner aptitudes (e.g., the learner's repertoire of knowledge, skills, and abilities) as a basis for assignment to instructional treatment and (2) learner achievement as a function of instructional treatment. Particular emphasis is given to the importance of homogeneity among items that comprise a test of achievement because a reliable test cannot be composed of nonhomogeneous items. Also emphasized is that an unreliable test cannot be useful for either diagnosis of evaluation. (Author/KC)