A cluster randomized trial involves social units or clusters of individuals, rather than individuals themselves, being randomised to different intervention groups. In such circumstances, individual patient level data are still collected. The units of randomization in these studies are diverse ranging from families, households, worksites, and hospitals to entire neighbourhoods or communities. The primary difference between a cluster and patient randomized trial is that participants within one cluster may be more alike (correlated) than participants in another cluster. The measure of this correlation is known as the intraclass correlation. As the correlation increases, a trial requires an increasing number of participants compared to a patient randomised trial (see Sample size justification ). See further reading and examples for more details. Whilst many characteristics of cluster randomized trials are in common with patient randomized trials, the adoption of a cluster randomized design has some implications for the design, conduct and analysis of the trial. Thus, most sections of this protocol support tool apply directly to cluster randomized trials. Special consideration needs to be given to cluster trials in the following sections (the specific issues are outlined in the relevant subsections):
Illustrative example - WELLINGTON trial
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Farms were randomised to receive a community health worker intervention to improve tuberculosis control in the agricultural sector of the Winelands Health District, South Africa. (WELLINGTON trial - go to protocol) |