Table 1.

Effective Sample Size and Power Holding mk Constant

Number of Physicians	Number of Patients	Total Number	ρ= 0.017		Power
k	m	(mk)	DE	ESS	tTest*
*Power to detect a minimum effect size of 0.5 when one half the clusters is randomly assigned to treatment and the other half to placebo, and mean responses are compared using a 2-sample t test made at the .05 level of significance.
m = number of subjects in a cluster; k= number of clusters; mk = total number of subjects in a clustered study; DE = design effect; and ESS = effective sample size.
4	32	128	1.527	84	61
8	16	128	1.255	102	70
16	8	128	1.119	114	75
32	4	128	1.051	122	78
64	2	128	1.017	126	79
128	1	128	1.000	128	80