Trait-Treatment Interaction (TTI), a research method for observing experimental effects of treatments on subjects of different aptitudes and learning characteristics, is suggested as an effective evaluation tool to provide evaluators and educators in compensatory education programs with information about which program is best for different kinds of learners. The premise of TTI research is that different instructional conditions work best when matched with selected learner traits, with "trait" defined as any characteristic of the learner that increases or impairs his probability of success in a given treatment. The methodology of Trait-Treatment Interaction (TTI) as outlined provides an alternative of the dilemma of control groups; and a proposed evaluation design is presented using Campbell's terminology of invited-accepted, invited-rejected, and uninvited. To tap the interaction of learner characteristics with instruction, the invited-accepted are tested and blocked or grouped on relevant learner characteristics, and then, within each block, randomly assigned to a treatment. The goal of this TTI factorial design is to develop alternative instructional programs for compensatory education which produce optimal educational payoff among pupils assigned differently to these programs on the basis of learner characteristics. (CS)