The familiar methods for sample size estimation used in randomized clinical trials (RCTs) require modifications for use in group- or cluster-randomized trials (GRTs) to account for the positive intraclass correlation (ICC) expected among members of the same group or cluster.6, 17, 19, 23, 37, 38 The specific modifications will depend on the design of the study, the scale of measurement for the outcome variable, and the analytic plan.
This section of the website provides a calculator to estimate sample size for a GRT. You will be asked to specify the type I error rate and desired power for the test of your intervention effect, the scale of measurement of your outcome variable, and the design and analytic plan for your trial. You will also be asked to provide estimates of the parameters needed to estimate the sample size required for your study.
There are nine steps for each sample size calculation. You may use the BACK button to go back to the previous step. You may also click on any of the completed steps in the list at the top of each page to go back to an earlier step. Information will be retained for each step only if you click on the SAVE AND CONTINUE button. Any information you have saved will be retained unless it is overwritten by a subsequent selection or entry. You can repeat this exercise as often as you wish to compare different designs and/or analytic plans, or change your parameter estimates, and you can print the results from each exercise.
The terminology and methods used to estimate sample size are based on work published in the first textbook on the design and analysis of group-randomized trials and updated in subsequent articles.30, 38, 41 These methods are equivalent to methods published by others and should give similar results.
As noted above, it is important for investigators to become familiar with the material on this website. It is even more important that they collaborate with a methodologist familiar with these issues, particularly in the development of the study design, analytic plan, and sample size calculations. Investigators may find it difficult to work through the sample size calculator on their own, and may have better experience by turning that task over to their methodologist.
Review a list of recent publications on sample size estimation methods for GRTs, and watch a recent Mind the Gap webinar, which demonstrates how to use this calculator.