American Journal of Epidemiology Vol. 144, No. 4: 425-433
Copyright © 1996 by The Johns Hopkins University School of Hygiene and Public Health
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Intraclass Correlation Estimates in a School-based Smoking Prevention Study
Outcome and Mediating Variables, by Sex and Ethnicity
From the Prevention Research Center (M/C 275), School of Public Health, University of Illinois at Chicago 850 West Jackson Boulevard, Suite 400, Chicago, IL 60607-3025
Most school-based smoking prevention studies employ designs in which schools or classrooms are assigned to different treatment conditions while observations are made on individual students. This design requires that the treatment effect be assessed against the between-school variance. However, the between-school variance is usually larger than the variance that would be obtained if students were individually randomized to different conditions. Consequently, the power of the test for a treatment effect is reduced, and it becomes difficult to detect important treatment effects. To assess the potential loss of power or to calculate appropriate sample sizes, investigators need good estimates of the intraclass correlations for the variables of interest. The authors calculated intraclass correlations for some common outcome variables in a school-based smoking prevention study, using a three-level model—i.e., students nested within classrooms and classrooms nested within schools. The authors present the intraclass correlation estimates for the entire data set, as well as separately by sex and ethnicity. They also illustrate the use of these estimates in the planning of future studies. Am J Epidemiol 1996; 144: 425–33.
adolescence; cluster analysis; epidemiologic methods; models, statistical; research design; smoking
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