Definition and rationale

Cluster randomised trials (CRCTs), sometimes called group randomised, field, community-based or place-based trials, involve the random allocation of existing groups of individuals to study arms. Group (‘cluster’) size can vary, ranging from families or classrooms to clinics or communities. Because many injury prevention interventions (eg, community education, mass media and legislation) are delivered to groups, CRCTs are an important methodology for evaluation studies in this field.

Members of clusters tend to respond to interventions in ways that are more similar to others in the same cluster than to members of different clusters, because, first, like people choose to join the same cluster (eg, schools, churches); second, cluster members have common exposures (eg, living on a busy street); and, third, cluster members interact with each other in ways that influence their responses (eg, sharing information).1 Thus, in a CRCT, participant outcomes are usually correlated within clusters, which affects both sample size requirements and statistical analysis. Even minimal intracluster correlation can substantially increase required sample size (see accompanying paper in this issue2). Except in rare instances when there is no intracluster correlation, CRCTs will therefore have reduced statistical efficiency relative to trials that individually randomise the same number of people.3

Because CRCTs are more complex to design, implement and analyse than individually randomised controlled trials, as we discuss below and in our companion paper,2 alternative designs should be carefully considered and use of a clustered design justified. Justifications fall broadly into two categories: scientific and logistical.4

Scientific justification

Cluster-level intervention and action

The intervention is delivered to and affects groups of people rather than, or in addition to, individuals. For example, one experimental school-based violence prevention programme established a local task force …