Intracluster correlation coefficients in cluster randomized trials: empirical insights into how should they be reported

In this paper, we describe the results of a survey to inform the appropriate reporting of the intracluster correlation coefficient (ICC) – the statistical measure of the clustering effect associated with a cluster randomized trial.We identified three dimensions that should be considered when reporting an ICC – a description of the dataset (including characteristics of the outcome and the intervention), information on how the ICC was calculated, and information on the precision of the ICC.This paper demonstrates the development of a framework for the reporting of ICCs. If adopted into routine practice, it has the potential to facilitate the interpretation of the cluster trial being reported and should help the development of new trials in the area.In evaluative health care research, the randomized controlled trial is generally considered the gold standard design for assessing the relative effectiveness of alternative interventions, as it ensures that selection bias and other common sources of bias are minimized [1]. In most of these trials, patients are allocated individually to the different treatments. It is being increasingly recognised, however, that there are many situations where randomizing by groups of individuals may be more appropriate [2,3]. For example, when assessing a dietary intervention, it is common to randomize families as an intact unit, to avoid the possibility of different members of the same family being assigned to different interventions. Trials that randomize by groups are known as cluster randomized trials [3,4].The adoption of a clustered design is not without cost, however, as the design and conduct of these trials requires special considerations. They are more complex to design, they require more participants than an individually randomized trial, and require more complex analysis. This added complexity arises primarily because observations on individuals within the same cluster may be correlated – that is, the outcomes for individuals wi