4 March 2019 - Cluster randomised trials randomise groups of individuals rather than individuals themselves to interventions. 

The groups might be communities, schools, workplaces, hospitals, or patients treated by a particular doctor. There are a number of reasons for the use of cluster trials as opposed to individually randomised trials. They may be the only available choice, as when a city is randomised to a mass intervention. 

Another reason is that the investigators may wish to reduce the risk of contamination, or it may be more effective, more convenient or cheaper to deliver an intervention to a group rather than to an individual.

Read Medical Journal of Australia article (subscription required)