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Supplemental Appendix
Supplemental Appendix. The Variance Inflation Factor (VIF)
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- Supplemental data: Appendix - PDF file, 1 page, 205 KB
The Article in Brief
David M. Thompson, and colleagues
Background Research conducted in practice-based research networks often randomizes interventions, not by individuals, but according to a natural clustering unit, such as the physician or the practice in which patients receive care. Researchers who conduct cluster-randomized studies must explicitly account for clustering at every stage of design and analysis to avoid underpowered studies. This requires good estimates of clustering effects in the form of intraclass correlation coefficients (ICCs). This study uses data from the Robert Wood Johnson Foundation's Prescription for Health program to estimate ICCs for demographic and behavioral variables and for physician and practice characteristics.
What This Study Found The authors analyzed data from 5,042 patients in 61 practices and 8 practice-based research networks. They found that ICCs for certain measures of health behavior and intent to change those behaviors are small, generally less than 0.1. Clustering is less evident for outcome variables than for other independent and process variables.
Implications
- Though small, the ICCs in this report are not trivial; if cluster sizes are large, even small levels of clustering, if unaccounted for, can reduce a study�s statistical power.