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%