Intraclass Correlation Coefficients Typical of Cluster-randomized Studies: Estimates from the Robert Wood Johnson Prescription for Health Projects

Purpose: Researchers who conduct cluster-randomized studies must account for clustering during study planning; failure to do so can result in insufficient study power. To plan adequately, investigators need accurate estimates of clustering in the form of intraclass correlation coefficients (ICCs).

Methods: We used data for 5,042 patients, from 61 practices in 8 practice-based research networks, obtained from the Prescription for Health program, sponsored by the Robert Wood Johnson Fund, to estimate ICCs for demographic and behavioral variables and for physician and practice characteristics. We used an approach similar to analysis of variance to calculate ICCs for binary variables and mixed models that directly estimated between- and within-cluster variances to calculate ICCs for continuous variables.

Results: ICCs indicating substantial within-practice clustering were calculated for age (ICC = 0.151), race (ICC = 0.265), and such behaviors as smoking (ICC = 0.118) and unhealthy diet (ICC = 0.206). Patients' intent-to-change behaviors related to smoking, diet, or exercise were less clustered (ICCs ≤0.007). Within-network ICCs were generally smaller, reflecting heterogeneity among practices within the same network. ICCs for practice-level measures indicated that practices within networks were relatively homogenous with respect to practice type (ICC = 0.29) and the use of electronic medical records (ICC = 0.23), but less homogenous with respect to size and rates of physician and staff turnover.

Conclusion: ICCs for patient behaviors and intent to change those behaviors were generally less than 0.1. Though small, such ICCs are not trivial; if cluster sizes are large, even small levels of clustering that is unaccounted for reduces the statistical power of a cluster-randomized study.