March 17, 2020: Cheat Sheet on the Intraclass Correlation Coefficient

The NIH Collaboratory Biostatistics and Study Design Core has created an Intraclass Correlation Coefficient (ICC) Cheat Sheet to provide an introductory description of the ICC, which is important for the design and analysis of cluster-randomized trials.

“The intraclass correlation coefficient (ICC) is a descriptive statistic that describes the extent to which outcomes 1) within each cluster are likely to be similar or 2) between different clusters are likely to be different from each other, relative to outcomes from other clusters. The ICC is an important tool for cluster-randomized pragmatic trials because this value helps determine the sample size needed to detect a treatment effect.” —from the ICC Cheat Sheet

The tool is a 2-page handout that can be used in trainings or classes regarding pragmatic clinical trials involving cluster randomization.

For more on the ICC, see the Intraclass Correlation section in the Living Textbook or this in-depth working document on the ICC from the Biostatistics and Study Design Core. If you have questions, feedback or suggestions regarding this tool, please contact us at