Methodological issues in the design and analysis of cluster randomized trials

Abstract

Randomized controlled trials are the accepted gold standard for evaluating the effects of interventions to improve health. In the majority of such trials, individuals are randomly allocated to the experimental conditions under study, for example, to treatment and control arms. However, in some situations it is more appropriate to randomly allocate groups of individuals to the treatment arms. These groups are referred to as clusters, and trials of this kind are known as cluster randomized trials (CRTs). Examples of clusters include schools, villages, workplaces, or health facilities, but there are many other possible choices. In some CRTs, all individuals within the selected clusters are automatically included. In others, there may be additional eligibility criteria. Similarly, the impact of the intervention may be measured in all individuals in the cluster, or in a random subsample. This chapter aims to discuss methodological issues that arise in the design and analysis of CRTs