Aptitude-treatment interaction (ATI) methods are designed to take individual differences into account systematically in treatment evaluation. This article reviews the general concepts of aptitude and ATI and summarizes lessons learned in ATI research on educational treatments that should help ATI research on psychotherapeutic treatments. Recommendations for research design and data analysis address problems of aptitude distributions, multivariate aptitude complexes, detective work with scatterplots, disattenuation, treatment and therapist characteristics, therapistclient matching, ecological validity, outcome variables, statistical power, aggregation, and person independence. Example studies and hypotheses about the nature of ATI processes are also included.

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This article reviews progress toward the development of a cognitive theory of aptitude for learning and presents descriptive and prescriptive goals for aptitude theories. Preliminary hypotheses about the nature of cognitive processes in aptitude for learning from instruction are reviewed. Twelve constituent points of the descriptive theory are presented. Some of these points are summary conclusions of much prior research, whereas others are less well supported at present. However, all contribute to the effort to describe learning and aptitude for learning in conformable terms. Finally, some prescriptive implications of the theory are discussed.The existence of individual differences in cognitive aptitude for learning from instruction is the most longstanding, wellestablished fact in educational psychology. This fact has been extensively studied and variously interpreted over the history of the field; it has been given its place in many theoretical models of school learning, both old and new, and a formidable measurement