Intra-cluster correlation coefficients from school-based cluster randomised trials of interventions for improving health outcomes in pupils

Background and Objectives To summarize intracluster correlation coefficient (ICC) estimates for pupil health outcomes from school-based cluster randomized trials (CRTs) across world regions and describe their relationship with study design characteristics and context. Methods School-based CRTs reporting ICCs for pupil health outcomes were identified through a literature search of MEDLINE (via Ovid). ICC estimates were summarized both overall and for different categories of study characteristics. Results Two hundred and forty-six articles reporting ICC estimates were identified. The median (interquartile range) ICC was 0.031 (0.011 to 0.08) at the school level (N = 210) and 0.063 (0.024 to 0.1) at the class level (N = 46). The distribution of ICCs at the school level was well described by the beta and exponential distributions. Besides larger ICCs in definitive trials than feasibility studies, there were no clear associations between study characteristics and ICC estimates. Conclusion The distribution of school-level ICCs worldwide was similar to previous summaries from studies in the United States. The description of the distribution of ICCs will help to inform sample size calculations and assess their sensitivity when designing future school-based CRTs of health interventions.