Client-treatment interaction in the study of differential change processes.

Research that combines correlational and experimental approaches in a search for aptitude-treatment interactions (ATI) is both inescapable and of potential benefit to the field. A number of epistemological and methodological issues regarding ATI research are discussed, ranging from the meaning of interaction to questions regarding the number of aptitudes and their types and the nature of interacting treatments. Despite the less than encouraging record of ATI research, it should not be abandoned, rather its purposes should be expanded. Instead of focusing on the pragmatic but hard-to-achieve goal of finding an optimal match between individual clients and specific therapies. ATI research should serve heuristic purposes. Theoretically derived and empirically tested interactions can clarify the change processes and mechanisms that make therapies differentially effective. Exploration of another type of interaction is recommended also: aptitude-outcome interaction (AOI), in which ostensibly identical therapies yield different outcomes through different mechanisms for different clients. The emphasis of AOI research is also more heuristic than pragmatic.