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

BMC Med Res Methodol. 2006 May 3:6:20. doi: 10.1186/1471-2288-6-20.


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

Methods: 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.

Results: 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).

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

Publication types

  • Research Support, N.I.H., Extramural

MeSH terms

  • Adult
  • Biomedical Research / methods*
  • Cluster Analysis
  • Data Interpretation, Statistical
  • Diabetes Mellitus / physiopathology
  • Diabetes Mellitus / therapy*
  • Female
  • Health Services Research / methods*
  • Humans
  • Information Systems*
  • Male
  • Primary Health Care / statistics & numerical data*
  • Surveys and Questionnaires
  • Vermont