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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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