Putting theory into practice: a cluster randomized trial with a small number of clusters.

Many of the difficulties encountered in the design, organization and analysis of cluster randomized trials arise from the dual nature of such trials; that is, they focus on both the cluster and the individual. A trial now in progress to compare three methods of promoting secondary prevention of coronary heart disease in primary care includes only 21 general practices, but 2142 patients, and thus contains the problems of both small and large samples. With only seven practices in each arm, the trial demanded carefully restricted randomization, may be difficult to analyse, and risks loss of power if one practice should drop out. At the same time, the large number of patients makes for an expensive and administratively complex study. The simultaneous demands of clarity and thoroughness point to an analysis at both cluster and individual level. With two different approaches, however, there may be difficulties of presentation, even if the results agree, and additional problems of interpretation if they do not. Finally, practical considerations may conflict with theoretical demands. Since the trial contained a service element, all patients with heart disease had to be included, even though it would have been more efficient to take only a sample of patients from some practices.