A brief history of the cluster randomised trial design

The cluster randomised trial is commonly considered a relatively new research study design.1–3 Here we trace to a few very early reports the idea of comparing interventions applied to groups of individuals, and through the evolution of this idea to the modern-day cluster randomised trial. This has been defined as a comparative study in which the units randomised are pre-existing (natural or self-selected) groups whose members have an identifiable feature in common, and in which outcomes are measured in all, or a representative sample of the individual members of the groups.1 Summaries of the reports of many of the examples of cluster randomised trials published before the methodological review of this research design by Donner et al.4 can be viewed in the James Lind Library (http://jameslindlibrary.org/topics/allocation-bias/cluster-allocation/ see Appendix for details of our literature search).