Heterogeneity of Treatment Outcomes Across Therapists and Sites in a Randomized Multicenter Psychotherapy Trial

Objective: Establishing robust evidence for the efficacy of psychotherapeutic treatments is fundamental to evidence-based medicine for patients with mental disorders. Cornerstones of testing the efficacy of psychotherapeutic treatments are randomized controlled trials (RCTs). These trials aim to control for many common sources of bias, such as selection effects and baseline imbalances between study groups. However, another challenge to establishing robust evidence in psychotherapy research is the heterogeneity in treatment outcomes due to therapists and clinical sites. While this has been frequently observed in naturalistic settings, therapist and site-related heterogeneity in treatment outcomes has been relatively understudied in RCTs. Method: The present study addresses this gap in a secondary data analysis, examining how strongly therapists and clinical sites differ in treatment outcomes and differential treatment effect (i.e., the difference in outcomes between treatment groups) within a large, multicenter RCT. We analyzed data from 255 patients treated by 79 therapists in nine study sites, that received two different active psychotherapeutic interventions. Results: We found small therapist-related (1.1%) and site-related (1.7%) variance in treatment outcome, while the site-related variance in differential treatment effect was comparatively large (12.5%). The variance in treatment outcome and differential treatment effect could only partially be explained by patient or therapist characteristics. Conclusion: These findings underscore the need to consider possible heterogeneity between clinical study sites in differential treatment effects in the evaluation of RCTs. Moreover, further research on the impact of study sites’ characteristics on treatment efficacy is needed. This could enhance our understanding of the efficacy of psychotherapeutic treatments in varying contexts.