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. In addition, a document illustrating example calculations for the variance of continuous, dichotomous, and count outcomes is provided.
Worked Examples for Detectable Difference
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:
- Unmatched and unstratified (PDF)
- A priori stratified (PDF)
- Post hoc stratified (PDF)
- A priori matched (PDF)
Examples based on an analysis of a net difference:
- Cohort Design
- Unmatched and unstratified (PDF)
- A priori stratified (PDF)
- Post hoc stratified (PDF)
- A priori matched (PDF)
- Cross-Sectional Design
- Unmatched and unstratified (PDF)
- A priori stratified (PDF)
- Post hoc stratified (PDF)
- A priori matched (PDF)
Worked Examples for Outcome Variance
The distribution of the outcome – continuous, dichotomous, or count – can determine its variance. Clicking the link below will take you to a PDF that presents how outcome variance is calculated based on user inputs as well as example calculations.