The familiar methods for sample size estimation used in randomized clinical trials (RCTs) require modifications for use in parallel group- or cluster-randomized trials (GRTs), individually randomized group-treatment (IRGT) trials, and stepped wedge group- or cluster-randomized trials (SWGRTs) to account for the positive intraclass correlation (ICC) expected among members of the same group or cluster (Campbell and Walters, 2014; Donner and Klar, 2000; Eldridge and Kerry, 2012; Hayes and Moulton, 2017; Moerbeek and Teerenstra, 2016; Murray, 1998). The specific modifications will depend on the design of the study, the expected distribution of the outcome variable, and the analytic plan.

The terminology and methods used to estimate sample size for GRTs and IRGTs are based on work published in the first textbook on the design and analysis of parallel group-randomized trials (Murray, 1998) and updated in subsequent articles (Janega et al., 2004; Murray et al., 2007; Pals et al., 2008). The terminology and methods used to estimate sample size for SWGRTs are based on several important articles (

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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 a better experience by turning that task over to their methodologist.

**Tips for Using the Sample Size Calculators**

- Information will be saved for each step only if you click the "Continue" button. Any information you save will be retained unless it is overwritten by a subsequent selection or entry.
- Use the "Back" button to return to the previous step.
- Click on any of the completed steps in the list on the left side of each page to go back to that step. Change your entry if you wish, and after clicking "Continue," you can move to any other step, including "Analysis."
- You can repeat this exercise as often as you wish to compare different designs and/or analytic plans or change your parameter estimates.
- You can print or download the results from each exercise.

Use this calculator to estimate sample sizes for parallel GRTs.

There are nine steps for each sample size calculation. You will be asked to specify the type I error rate and desired power for the test of your intervention effect, the expected distribution 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. Please note that this sample size calculator assumes the same variance components, ICCs, and group sizes in both study conditions; those assumptions are appropriate for most parallel GRTs.

**Launch the Parallel GRT Sample Size Calculator**

**Additional Resources**

For more information, please review a list of recent publications on sample size estimation methods for parallel GRTs or watch a Methods: Mind the Gap webinar, which demonstrates how to use the parallel GRT calculator.

Use this calculator to estimate sample size for an IRGT trial with clustering in only one arm. If you expect to have clustering in both arms, you can use the GRT Sample Size Calculator, because GRTs always have clustering in both arms. Please note however, that the GRT Sample Size Calculator assumes the same variance components, ICCs, and group sizes across study conditions, which may not always be appropriate for an IRGT with clustering in both arms ( ).

There are eight steps for each sample size calculation for an IRGT trial. You will be asked to specify the type I error rate and desired power for the test of your intervention effect, the expected distribution 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 calculate the sample size required for your study.

**Launch the IRGT Sample Size Calculator**

**Additional Resources**

For more information, please review a list of recent publications on sample size estimations methods for IRGT trials or watch a Methods: Mind the Gap webinar, which provides useful information on their design, analysis, and sample size estimations.

Use this calculator to estimate sample size for SWGRTs.

There are seven steps for each sample size calculation. You will be asked to specify the type I error rate and desired power for the test of your intervention effect, the expected distribution 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. Please note that this sample size calculator assumes the same variance components, ICCs, autocorrelations, and group sizes in all sequences.

**Launch the SWGRT Sample Size Calculator**

**Additional Resources**

For more information, please review a list of recent publications on sample size estimations methods for SWGRTs or watch a Methods: Mind the Gap webinar, which provides an overview of statistical models used in the design and analysis of SWGRTs.

**Suggested citation**: Research Methods Resources: National Institutes of Health. [Accessed Month Day, Year]. Available from: https://researchmethodsresources.nih.gov/.