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Cluster Unit Randomized Trials

19. Summary

The purpose of this chapter is to provide a basic understanding of methodological issues that must be addressed when investigators decide to randomize intact social units, or clusters of individuals, to different intervention groups. Foremost among these is the justification for randomizing clusters rather than individuals given the loss of statistical efficiency that inevitably arises. The impact of cluster randomization on sample size estimation, the choice of an experimental design, and the approach to the statistical analysis are discussed in detail. Consideration is also given to the unique ethical issues that arise when clusters are selected as the unit of randomization. The chapter closes with suggested guidelines for the reporting of trial results.