Process transforms inputs to determine outcomes: Therapists are responsible for managing process.

The variance in outcomes for psychotherapy patients is not partitionable into components that are independent contributions of treatments, therapists, and patients. If these inputs did not influence one another over the course of psychotherapy, they could be independent and so have additive main-effects or interaction-effects on outcomes. But that is impossible because they do influence one another and therapists are responsible for actively managing the psychotherapy process by repeatedly adjusting these inputs toward optimally influencing one another. The consequent interdependence of these inputs within the therapy process needs to be reflected in the design and analysis of psychotherapy outcome studies, as it presently is not, if we are to learn who is adequate for treating whom, how, and why so.