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Table 3 Power for detecting a difference between the intervention and control group, depending on effect size and intra-class correlation (34 sites, average 6.4 subjects per site)

From: Chronic pain self-management for older adults: a randomized controlled trial [ISRCTN11899548]

Intra-class correlation Effective sample size Effect size (the difference in means between the two groups, divided by the within-group standard deviation)
   .40 .50 .60 .70
0.00 218 84% 96% 99% 100%
0.05 171 74% 90% 97% 100%
0.10 141 65% 84% 94% 98%
0.20 105 52% 71% 86% 94%
0.30 83 43% 61% 77% 88%
0.50 59 32% 46% 61% 75%
1.00 34 20% 29% 40% 51%