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The design and use of cluster randomised controlled trials in evaluating injury prevention interventions: part 2. Design effect, sample size calculations and methods for analysis
  1. Carol Coupland1,
  2. Carolyn DiGuiseppi2,3
  1. 1Division of Primary Care, University of Nottingham, UK
  2. 2Colorado School of Public Health, University of Colorado Denver, Aurora, Colorado, USA
  3. 3Colorado Injury Control Research Center, Fort Collins, Colorado, USA
  1. Correspondence to Carol Coupland, Division of Primary Care, Tower Building, University Park, Nottingham NG7 2RD, UK; Carol.coupland{at}

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Elsewhere1 we have described the rationale for carrying out cluster randomised controlled trials (CRCTs) in injury prevention and discussed key issues relating to the design and ethical conduct of such studies. In this companion paper we focus on sample size calculations for cluster randomised trials and on the methods for statistical analyses of these studies.

Design effect and the intracluster correlation coefficient

As previously reported in our companion paper,1 a major disadvantage of CRCTs is that they generally require a larger sample size than do individually randomised trials. This increase in sample size can be quite substantial, depending on the size of the clusters being randomised and the degree of similarity of outcomes among members of the same cluster. The key measure of this similarity is called the intracluster or intraclass correlation coefficient (ICC), often denoted as ρ. This measure reflects the correlation between outcome values in members of the same cluster. If all members of the same cluster have identical values of the outcome measure, the ICC is equal to 1. An ICC of 0 would indicate that there is no correlation in outcome values between members of the same cluster, such that a member of any particular cluster is likely to have values that are no more similar to those of another member of the same cluster than they are to a member of a different cluster. Negative values of the ICC would occur where outcomes for members in the same cluster are less alike than they are for members in different clusters, but this is unlikely to be the case in CRCTs, although estimates of the ICC may be negative due to sampling error. Assuming that the ICC has only …

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  • Funding CD was funded in part by a grant from the National Center for Injury Prevention and Control, Centers for Disease Control and Prevention, Atlanta, GA.

  • Competing interests None.

  • Provenance and peer review Commissioned; externally peer reviewed.

  • i ICC=σb2/(σb2w2) where σb2 is the between-cluster component of variance and σw2 is the within-cluster component of variance.

  • ii An extension of the CONSORT guidelines for the reporting of individually randomised controlled trials, which addresses the unique aspects of reporting CRCTs, has been published.15 At the end of each section, we provide guidelines and examples for reporting the issue discussed, based on the extended CONSORT guidelines.

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