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)

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%

Ersek et al. BMC Geriatrics 2004 4:7   doi:10.1186/1471-2318-4-7