Randomization: Beyond the closurization principle.

Many cluster randomized trials have relatively few numbers of clusters to be randomized. When baseline cluster-level covariates are available prior to randomization, the set of potential allocations can be restricted so as to ensure balance across study arms. This article discusses why and how restrictions can be made, and the ramifications of so doing. The Fisher-Bailey validity is explained, and examples are given regarding the tradeoff between balance and validity.