A Cross-National Comparison of Intra-Class Correlation

The plausible range of intra-class correlation coefficient (ICC) is essential for both a priori sample size calculations in planning cluster-randomized trials and statistical adjustments of misaligned analysis of clustered data in meta-analytic studies. Recent efforts to create databases for ICC in educational achievement outcomes are based on the studies published only in the US and Europe. The current study aims to extend the existing information for the plausible range of ICC values in educational achievement outcomes to a global scale by examining the distributional characteristics of two-level unconditional ICC estimates across countries participating in two international studies, TIMSS and PIRLS. The findings suggest large variability in the unconditional ICC estimates across countries, and current standards do not apply to every country. Researchers should look for country-specific ICC estimates in planning cluster-randomized trials and in comparing studies across countries.