Designing Cluster Randomized Trials: A Practical Guide for Investigators

An important consideration in the design stage of randomized controlled trials is whether individuals within each site should or can be randomized to study arms (an individually randomized controlled trial) or whether entire sites should be randomized (a cluster randomized trial) where the clusters are sites. Recently, cluster randomized trials have grown in popularity; however, investigators have expressed a need for guidelines related to selection of the best design-especially when considering more complex designs such as stepped wedge. This commentary attempts to provide such guidance.

In this commentary, we address common misconceptions surrounding the appropriate use of cluster randomization and crossover designs (such as the stepped wedge design). The intracluster correlation coefficients for different levels of clustering are presented, and the impact of different designs on sample size discussed. We provide practical recommendations that are accessible to investigators from all disciplines, regardless of their level of statistical training. On the basis of these recommendations, we present a flowchart to help researchers identify a suitable trial design for their study. In addition, we provide a table of commonly used statistical definitions related to cluster randomized trials and a table of design considerations for more complex designs to support effective consultation with a statistician during the design phase.

On the basis of these guidelines, it is recommended that researchers prioritize the use of individual randomization and a parallel trial design when the goal of the study is to assess the efficacy or effectiveness of an intervention.