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. Author manuscript; available in PMC: 2008 Oct 15.
Published in final edited form as: Epidemiology. 2007 Sep;18(5):552–560. doi: 10.1097/ede.0b013e3181200199

FIGURE 1.

FIGURE 1

Power for detecting a difference in prevalence of past 30-day alcohol use of 0.10 between control and intervention conditions with a pretest–posttest nested cross-sectional design with n = 48 and m = 15. The diagonal lines indicate pairs of within-time and between-time intracluster correlation values that give a fixed level of power and that are useful values of correlations for planning a future trial as they lie in the ellipse that defines a 95% joint confidence region for the intracluster correlations.