Objective: To review the therapist effects literature since Baldwin and Imel's (2013) review.
Method: Systematic literature review of three databases (PsycINFO, PubMed and Web of Science) replicating Baldwin and Imel (2013) search terms. Weighted averages of therapist effects (TEs) were calculated, and a critical narrative review of included studies conducted.
Results: Twenty studies met inclusion criteria (3 RCTs; 17 practice-based) with 19 studies using multilevel modeling. TEs were found in 19 studies. The TE range for all studies was 0.2% to 29% (weighted average = 5%). For RCTs, 1%-29% (weighted average = 8.2%). For practice-based studies, 0.2-21% (weighted average = 5%). The university counseling subsample yielded a lower TE (2.4%) than in other groupings (i.e., primary care, mixed clinical settings, and specialist/focused settings). Therapist sample sizes remained lower than recommended, and few studies appeared to be designed specifically as TE studies, with too few examples of maximising the research potential of large routine patient datasets.
Conclusions: Therapist effects are a robust phenomenon although considerable heterogeneity exists across studies. Patient severity appeared related to TE size. TEs from RCTs were highly variable. Using an overall therapist effects statistic may lack precision, and TEs might be better reported separately for specific clinical settings.
Keywords: Multilevel modeling; Practice-based studies; RCTs; Systematic review; Therapist effects.
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