Worked Examples: Explaining the Results of the Parallel GRT Sample Size Calculator

Below are worked examples that illustrate how the results are calculated once you click "Calculate Results" at the end of the eighth step (“Analysis”) of the group-randomized trial (GRT) sample size calculator

The 12 worked examples reflect the 12 primary variations that are addressed in this sample size calculator. These 12 variations are a function of the two factors that dictate the formula used to estimate sample size—the analytic approach, and the use of stratification and matching. Users select among two analytic approaches (analysis of a simple difference vs. analysis of a net difference; within net difference, cohort vs cross-section) and four approaches to stratification and matching (unmatched and unstratified, a priori stratified, post hoc stratified, and a priori matched). 

Separately, users make selections for the: Type I error rate; power; expected distribution of the primary outcome; intraclass correlation; number of members or participants per group or cluster; expected benefit, if any, from regression adjustment for covariates; and magnitude of the intervention effect. These choices provide the parameter estimates used in the formulae but do not dictate the structure of the formulae.

Clicking any of the 12 links below will take you to a PDF that presents the formula for the detectable difference for that variation and walks you through an example, showing how the detectable difference is calculated. You can open these files in any combination, save them, or print them, as they may be of help to you.

Examples based on an analysis of a simple difference:

Examples based on an analysis of a net difference:

Last updated on November 23, 2021