Are high-performing therapists both effective and consistent? A test of therapist expertise.

Objective: Therapist effectiveness has primarily been defined as being the aggregate of the client therapy outcomes within a therapist's caseload. It may seem intuitive that the most skilled therapists are both effective (in the way defined above) and consistent in facilitating positive outcomes across their clients; however, this premise has not been fully tested. The present study sought to empirically examine this question in a large, multisite, geographically diverse sample. We first computed a consistency variable and an aggregate outcome variable for each therapist among a subset of each individual therapist's caseload (the first 30 clients per therapist within our data set). We then utilized this consistency score and aggregate outcome score to predict the therapy outcomes of their remaining clients. Clients' pretreatment severity scores were also included as a moderator of the association between therapist consistency, therapist aggregate outcome, and client outcomes. Method: The sample included 27,778 clients who were treated by 275 therapists. At the start of each session, clients completed the Behavioral Health Measure-20 as a measure of psychological functioning. Results: Polynomial regression and response surface analysis indicated a discrepant effect, such that subsequent clients' outcomes were highest when therapists' aggregate outcome with their first 30 clients was high and the consistency in the outcomes of their first 30 clients was also high. This relationship was not moderated by clients' pretreatment severity. Conclusion: Therapists' expertise consists of both high performance and consistency. Therapists who achieved better outcomes consistently were top performers with their subsequent clients.