title: Intraclass correlation coefficients from three cluster randomised controlled trials in primary and residential health care.
abstract: OBJECTIVE
  This paper provides intraclass correlation coefficients (ICCs) for estimation of sample size inflation required in future cluster randomised trials in primary or residential care.
  
  
  METHODS
  Three cluster randomised trials were conducted among middle-aged and older adults in primary care and residential care in Australia and New Zealand between 1995 and 2002. Baseline means or proportions, mean change, and ICCs with their standard errors and 95% confidence intervals are reported for outcome variables used in the three studies. The ICCs were estimated from a one-way random effects model using the analysis of variance method.
  
  
  RESULTS
  ICCs for quality of life and psychological variables in the primary care studies were low (below 0.018). ICCs for clinical and physical activity variables ranged from 0 to 0.08. ICCs for health and functional status in residential care for the elderly were high, ranging from 0.025 to 0.514.
  
  
  CONCLUSIONS
  The magnitude of the intraclass correlation varies with the venue of the trial, the outcome variables used, and the expected effect of the intervention. However, the intraclass correlations provided will be useful for more appropriate planning of residential and primary care-based trials in the future.
authors.affiliation: Department of General Practice, Wellington School of Medicine, University of Otago, New Zealand. c.elley@wnmeds.ac.nz
authors.forename: C Raina
authors.initials: CR
authors.lastname: Elley
authors.forename: Ngaire
authors.initials: N
authors.lastname: Kerse
authors.forename: Patty
authors.initials: P
authors.lastname: Chondros
authors.forename: Elizabeth
authors.initials: E
authors.lastname: Robinson
mesh_terms: D000328:Adult; D000368:Aged; D040561:Assisted Living Facilities / Q000706:statistics & numerical data*; D016000:Cluster Analysis; D006305:Health Status Indicators; D006801:Humans; D008875:Middle Aged; D009520:New Zealand / Q000453:epidemiology; D011320:Primary Health Care / Q000706:statistics & numerical data*; D018401:Sample Size; D014739:Victoria / Q000453:epidemiology