The stepped wedge cluster randomised trial: rationale, design, analysis, and reporting

The stepped wedge cluster randomised trial is a novel research study design that is increasingly being used in the evaluation of service delivery type interventions. The design involves random and sequential crossover of clusters from control to intervention until all clusters are exposed. It is a pragmatic study design which can reconcile the need for robust evaluations with political or logistical constraints. While not exclusively for the evaluation of service delivery interventions, it is particularly suited to evaluations that do not rely on individual patient recruitment. As in all cluster trials, stepped wedge trials with individual recruitment and without concealment of allocation (or blinding of the intervention) are at risk of selection biases. In a stepped wedge design more clusters are exposed to the intervention towards the end of the study than in its early stages. This implies that the effect of the intervention might be confounded with any underlying temporal trend. A result that initially might seem suggestive of an effect of the intervention may therefore transpire to be the result of a positive underlying temporal trend. Sample size calculations and analysis must make allowance for both the clustered nature of the design and the confounding effect of time. The stepped wedge cluster randomised trial is an alternative to traditional parallel cluster studies, in which the intervention is delivered in only half the clusters with the remainder functioning as controls. When the clusters are relatively homogeneous (that is, the intra-cluster correlation is small), parallel studies tend to deliver better statistical performance than a stepped wedge trial. However, if substantial cluster-level effects are present (that is, larger intra-cluster correlations) or the clusters are large, the stepped wedge design will be more powerful than a parallel design, even one in which the intervention is preceded by a period of baseline control observations.