Understanding processes of change: how some patients reveal more than others-and some groups of therapists less-about what matters in psychotherapy.

Objective: We identify difficulties researchers encounter in psychotherapy process-outcome investigations, and we describe several limitations of the popular "variance accounted for" approach to understanding the effects of psychotherapy.

Methods & Results: Using data simulations, we show how the expected correlation between an excellent measure of therapy quality and outcome would be surprisingly small (approximately .25) under conditions likely to be common in psychotherapy research. Even when we modeled conditions designed to increase the likelihood that strong process-outcome relationships would be observed, we found that the expected correlations were still only in the modest range (.38-.51).

Conclusions: We discuss the implications of our analysis for the interpretation of process-outcome findings as well as for design considerations in future investigations.