ABSTRACT
With cluster randomized trials, complete groups of subjects are randomized to treatments and all members of a given group receive the same treatment condition. Such trials are variously known as group randomized trials, placebased trials and community intervention trials. They are conducted in many disciplines in the social and medical sciences and examples are trials in general practice with patients nested within practices, trials in organizational studies with employees nested within worksites and school-based substance abuse prevention and cessation trials with pupils nested within schools.
