• Gerard J P van Breukelen and Math J J M Candel.
• Department of Methodology and Statistics, CAPHRI School for Public Health and Primary Care, Maastricht University, Maastricht, The Netherlands. gerard.vbreukelen@maastrichtuniversity.nl
• J Clin Epidemiol. 2012 Nov 1; 65 (11): 1212-8.

ObjectiveSimple guidelines for calculating efficient sample sizes in cluster randomized trials with unknown intraclass correlation (ICC) and varying cluster sizes.MethodsA simple equation is given for the optimal number of clusters and sample size per cluster. Here, optimal means maximizing power for a given budget or minimizing total cost for a given power. The problems of cluster size variation and specification of the ICC of the outcome are solved in a simple yet efficient way.ResultsThe optimal number of clusters goes up, and the optimal sample size per cluster goes down as the ICC goes up or as the cluster-to-person cost ratio goes down. The available budget, desired power, and effect size only affect the number of clusters and not the sample size per cluster, which is between 7 and 70 for a wide range of cost ratios and ICCs. Power loss because of cluster size variation is compensated by sampling 10% more clusters. The optimal design for the ICC halfway the range of realistic ICC values is a good choice for the first stage of a two-stage design. The second stage is needed only if the first stage shows the ICC to be higher than assumed.ConclusionEfficient sample sizes for cluster randomized trials are easily computed, provided the cost per cluster and cost per person are specified.Copyright © 2012 Elsevier Inc. All rights reserved.

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