title: Intraclass correlation associated with therapists: estimates and applications in planning psychotherapy research.
abstract: 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.
authors.affiliation: Department of Psychology, Brigham Young University, Provo, Utah, USA. scott_baldwin@byu.edu
authors.forename: Scott A
authors.initials: SA
authors.lastname: Baldwin
authors.forename: David M
authors.initials: DM
authors.lastname: Murray
authors.forename: William R
authors.initials: WR
authors.lastname: Shadish
authors.forename: Sherri L
authors.initials: SL
authors.lastname: Pals
authors.forename: Jason M
authors.initials: JM
authors.lastname: Holland
authors.forename: Jonathan S
authors.initials: JS
authors.lastname: Abramowitz
authors.forename: Gerhard
authors.initials: G
authors.lastname: Andersson
authors.forename: David C
authors.initials: DC
authors.lastname: Atkins
authors.forename: Per
authors.initials: P
authors.lastname: Carlbring
authors.forename: Kathleen M
authors.initials: KM
authors.lastname: Carroll
authors.forename: Andrew
authors.initials: A
authors.lastname: Christensen
authors.forename: Kari M
authors.initials: KM
authors.lastname: Eddington
authors.forename: Anke
authors.initials: A
authors.lastname: Ehlers
authors.forename: Daniel J
authors.initials: DJ
authors.lastname: Feaster
authors.forename: Ger P J
authors.initials: GP
authors.lastname: Keijsers
authors.forename: Ellen
authors.initials: E
authors.lastname: Koch
authors.forename: Willem
authors.initials: W
authors.lastname: Kuyken
authors.forename: Alfred
authors.initials: A
authors.lastname: Lange
authors.forename: Tania
authors.initials: T
authors.lastname: Lincoln
authors.forename: Robert S
authors.initials: RS
authors.lastname: Stephens
authors.forename: Steven
authors.initials: S
authors.lastname: Taylor
authors.forename: Chris
authors.initials: C
authors.lastname: Trepka
authors.forename: Jeanne
authors.initials: J
authors.lastname: Watson
mesh_terms: D002986:Clinical Trials as Topic / Q000379:methods*; D006801:Humans; D017063:Outcome Assessment, Health Care; D011613:Psychotherapy / Q000379:methods*; D015203:Reproducibility of Results; D012107:Research Design; D016896:Treatment Outcome