Table 1

Sample sizes required for 90% power and 1% two-sided alpha: main effects. Intervention A target difference = 0.35 standard deviations (SDs), total sample size = 486 (243 allocated to Intervention A, 243 allocated to the relevant control). Intervention B target difference = 0.3 SDs, total sample size = 664 (332 allocated to Intervention B, 332 allocated to the relevant control). A total sample size of n = 664 participants yields 90% power to detect differences of 0.3 SDs for Intervention B and 97% power to detect differences of 0.35 SDs for Intervention A.

To detect the same target differences with the same power and alpha in a three-arm parallel group trial would require 907 participants: 243 allocated to Intervention A, and 332 allocated to each of Intervention B and control.

Montgomery et al. BMC Medical Research Methodology 2003 3:26   doi:10.1186/1471-2288-3-26