Aptitude-treatment interaction as a framework for research on individual differences in psychotherapy.

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 lesions 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, therapist-client matching, ecological validity, outcome variables, statistical power, aggregation, and person independence. Example studies and hypotheses about the nature of ATI processes are also included.