The American Psychological Association's Presidential Task Force on Evidence-Based Practice articulates a decision-making process based on the integration of the best available research, clinical expertise, and patient characteristics. The treatment method, the individual psychotherapist, the treatment relationship, and the patient are all vital contributors to successful psychotherapy. Several studies have shown that the variation in treatment effect among psychotherapists using the same method is greater than the variation in treatment effect among different methods. This outcome pleads for a comprehensive psychotherapist-related “g-factor” of psychotherapy. In terms of decision-making for health-care institutions and clinical practices, it would seem wise not to separate the psychotherapist, the patient, or the relationship from the method as there seem to be extensive effects relative to the interaction among these parameters. Parallel to a growing knowledge of evidence-based methods, there is still a decisive lack of scientific knowledge about these interactions.