Intraclass correlation associated with therapists: Estimates and applications in planning psychotherapy research

Scott A. Baldwin, David M. Murray, William R. Shadish, Sherri L. Pals, Jason M. Holland, Jonathan S. Abramowitz, Gerhard Andersson, David C. Atkins, Per Carlbring, Kathleen M. Carroll, Andrew Christensen, Kari M. Eddington, Anke Ehlers, Daniel J. Feaster, Ger P.J. Keijsers, Ellen Koch, Willem Kuyken, Alfred Lange, Tania Lincoln, Robert S. StephensSteven Taylor, Chris Trepka, Jeanne Watson

Research output: Contribution to journalArticlepeer-review

49 Scopus citations


It is essential that outcome research permit clear conclusions to be drawn about the efficacy of interventions. The common practice of nesting therapists within conditions can pose important methodological challenges that affect interpretation, particularly if the study is not powered to account for the nested design. An obstacle to the optimal design of these studies is the lack of data about the intraclass correlation coefficient (ICC), which measures the statistical dependencies introduced by nesting. To begin the development of a public database of ICC estimates, the authors investigated ICCs for a variety outcomes reported in 20 psychotherapy outcome studies. The magnitude of the 495 ICC estimates varied widely across measures and studies. The authors provide recommendations regarding how to select and aggregate ICC estimates for power calculations and show how researchers can use ICC estimates to choose the number of patients and therapists that will optimize power. Attention to these recommendations will strengthen the validity of inferences drawn from psychotherapy studies that nest therapists within conditions.

Original languageEnglish (US)
Pages (from-to)15-33
Number of pages19
JournalCognitive Behaviour Therapy
Issue number1
StatePublished - Mar 2011


  • Intraclass correlation
  • Power
  • Psychotherapy research
  • Statistical dependence
  • Therapist effects

ASJC Scopus subject areas

  • Clinical Psychology


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