Tools

The familiar methods for sample size estimation used in randomized clinical trials (RCTs) require modifications for use in group- or cluster-randomized trials (GRTs) and individually randomized group-treatment (IRGT) trials 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 are based on work published in the first textbook on the design and analysis of group-randomized trials (Murray, 1998) and updated in subsequent articles (Janega et al., 2004 ; Murray et al., 2007 ; Pals et al., 2008b; Pals et al., 2008a).

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

  • 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 an earlier step.
  • Information will be saved for each step only if you click the "Continue" button. Any information you 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.
  • You can print or download the results from each exercise.

Use this calculator to estimate sample sizes for 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 GRTs.

 

Launch the GRT Sample Size Calculator

 

Additional Resources

For more information, please review a list of recent publications on sample size estimation methods for GRTs or watch a Methods: Mind the Gap webinar, which demonstrates how to use the 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 (Roberts and Roberts, 2005).

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.

 

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

Last updated on April 8, 2021