COMPARATIVE OUTCOME EVALUATIONS OF PSYCHOTHERAPIES: GUIDELINES FOR ADDRESSING EIGHT LIMITATIONS OF THE GOLD STANDARD OF CAUSAL INFERENCE

Although randomized experiments represent the gold standard in causal research, their limitations are easily overlooked, especially in field experiments. Although discussions of the limitations of field experiments typically detail problems of design implementation, field experiments have limitations even if their randomized designs are successfully implemented. This is particularly true of comparative evaluations of psychotherapy programs. Accordingly, this article focuses on eight limitations that pervade such comparative treatment research: (a) Direct and indirect effects are conflated; (b) the investigation of interactions is limited; (c) attention is diverted from background conditions; (d) temporal factors complicate causal inferences; (e) the stable-unit-treatment-value assumption is easily violated; (f) therapist effects can confound therapy method effects; (g) differential effects of treatment methods are small; and (h) generalizability of treatment research findings is indeterminate. In addition to explaining each limitation, the author proposes research strategies for addressing it. The author concludes that attention to these 8 limitations and to the various strategies for overcoming them can increase the causal contribution of experimental evaluations of psychotherapies.