Although psychotherapy involves the interaction of client and therapist, mutual influence is not typically considered as a source of variability in therapist adherence and competence in providing treatments assessed in clinical trials. We examined variability in therapist adherence and competence in Motivational Enhancement Therapy (MET) both within and between caseloads in a large multi-site clinical trial. Three-level multilevel models (repeated measures, nested within clients, nested with therapists) indicated significant variability both within and between therapists. There was as much and sometimes more variability in MET adherence and competence within therapist caseloads than between therapists. Variability in MET adherence and competence within caseloads was not consistently associated with client severity of addiction at baseline. However, client motivation at the beginning of the session and days of use during treatment were consistent predictors of therapist adherence and competence. Results raise questions about the nature of therapist adherence and competence in treatment protocols. Accordingly, future analysis of clinical trials should consider the role of mutual influence in measures of therapist performance.
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