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

BACKGROUND Increasingly, researchers are recognizing that there are many situations where the use of a cluster randomized trial may be more appropriate than an individually randomized trial. Similarly, the need for appropriate standards of reporting of cluster trials is more widely acknowledged. METHODS 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. RESULTS 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. CONCLUSIONS 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.