Cluster randomized trial

A cluster randomised controlled trial is a type of randomised controlled trial in which groups of subjects (as opposed to individual subjects) are randomised. Cluster randomised controlled trials are also known as cluster randomised trials,group-randomised trials, and place-randomized trials.

A 2004 bibliometric study documented an increasing number of publications in the medical literature on cluster randomised controlled trials since the 1980s. Advantages of cluster randomised controlled trials over individually randomised controlled trials include the ability to study interventions that cannot be directed toward selected individuals (e.g., a radio show about lifestyle changes) and the ability to control for "contamination" across individuals (e.g., one individual's changing behaviors may influence another individual to do so).

Disadvantages compared with individually randomised controlled trials include greater complexity in design and analysis, and a requirement for more participants to obtain the same statistical power. Specifically, the cluster randomised designs introduce dependence (or clustering) between individual units sampled. An example would be an educational intervention in which schools are randomised to one of several new teaching methods. When comparing differences in outcome achieved under the new methods, researchers must account for the fact that two students sampled from a single school are more likely to be similar (in terms of outcomes) than two students sampled from different schools. Multilevel or similar statistical models are typically used to correct for this non-independence.

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