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Open Access Research article

Intra-cluster correlation coefficients in adults with diabetes in primary care practices: the Vermont Diabetes Information System field survey

Benjamin Littenberg* and Charles D MacLean

Author Affiliations

General Internal Medicine, University of Vermont, 371 Pearl Street, Burlington, VT, 05401, USA

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BMC Medical Research Methodology 2006, 6:20  doi:10.1186/1471-2288-6-20

Published: 3 May 2006



Proper estimation of sample size requirements for cluster-based studies requires estimates of the intra-cluster correlation coefficient (ICC) for the variables of interest.


We calculated the ICC for 112 variables measured as part of the Vermont Diabetes Information System, a cluster-randomized study of adults with diabetes from 73 primary care practices (the clusters) in Vermont and surrounding areas.


ICCs varied widely around a median value of 0.0185 (Inter-quartile range: 0.006, 0.037). Some characteristics (such as the proportion having a recent creatinine measurement) were highly associated with the practice (ICC = 0.288), while others (prevalence of some comorbidities and complications and certain aspects of quality of life) varied much more across patients with only small correlation within practices (ICC<0.001).


The ICC values reported here may be useful in designing future studies that use clustered sampling from primary care practices.