OBJECTIVE: An important question in implementation/dissemination research is whether the efficacy of a given treatment varies in part based on the therapist delivering the treatment. This study sought to provide practical guidance to researchers in the field of eating disorders for building measurement of therapist effects into the design of a typical, relatively small randomized controlled trial (RCT).

METHOD: Using assumptions based on past trials of eating disorder treatments, Monte Carlo simulations were used to examine 12 different scenarios based on crossing the number of therapists (between two and five) and the estimated therapist effect size (small, medium, and large). Patient sample size and study design were held constant.

RESULTS: There was reasonable power (>/=70%) to detect the therapist effect with three or four therapists and a large effect size.

DISCUSSION: Several practical implications for testing therapist effects in RCT are discussed. (c) 2012 by Wiley Periodicals, Inc. Int J Eat Disord 2012.