Center for Health Policy and Research (CHPR) Publications


How many therapists? Practical guidance on investigating therapist effects in randomized controlled trials for eating disorders

UMMS Affiliation

Center for Health Policy and Research; Department of Quantitative Health Sciences

Publication Date


Document Type



Eating Disorders; Psychotherapy


Health Services Administration | Health Services Research | Public Health


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.

DOI of Published Version



Int J Eat Disord. 2012 Jul;45(5):670-6. doi: 10.1002/eat.22009. Epub 2012 Mar 12. Link to article on publisher's site

Journal/Book/Conference Title

The International journal of eating disorders

Related Resources

Link to Article in PubMed