cluster randomization

Definition

The cluster RCT is an alternative trial design in which the study participants are randomized as entire groups or “clusters” rather than as individuals, as is the case with conventional RCTs. Cluster RCTs can be used to measure the community effect of an intervention, to review survival analysis of a population-based treatment, or to evaluate an educational intervention. The number of clusters required for the study is determined by sample size calculations designed specifically for cluster RCTs. By changing the unit or level of randomization from an individual to a group, the statistical analysis must subsequently be adapted.

A CRT might be preferred when there is a significant potential for contamination in the study. Contamination occurs when aspects of an intervention are adopted by members of the group that was randomized to not receive that intervention. (See also "What Is Contamination, and Why Does it Matter?" immediately below).

What Is Contamination, and Why Does it Matter?

The most compelling reason to randomize at the cluster level rather than at the individual level is the potential for contamination, whereby participants within a cluster are likely to be treated similarly and hence exhibit similar outcomes.

When contamination occurs during a clinical trial, it dilutes the observed differences between comparators and can affect the reliability and validity of the study.

Case Examples of Contamination

Participants who share the same provider in a trial comparing weight-loss strategies may meet each other in the waiting room and communicate about their respective strategies, or the provider might not be able to adapt to coaching differently depending on the randomization. Some participants in each group might even adopt elements of both strategies, and neither group would demonstrate the impact of its intended strategy. Randomization at the provider level, with each provider coaching only a single strategies, would reduce the risk of contamination.