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.
Campbell
Marion K
MK
Health Services Research Unit, University of Aberdeen, Aberdeen, UK. m.k.campbell@abdn.ac.uk
Grimshaw
Jeremy M
JM
Elbourne
Diana R
DR
D016000:Cluster Analysis; D003627:Data Interpretation, Statistical; D036262:Empirical Research; D006801:Humans; D016032:Randomized Controlled Trials as Topic / Q000592:standards*; D018401:Sample Size