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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.

From: Design, analysis and presentation of factorial randomised controlled trials

   Intervention B  
   YES NO TOTAL
Intervention A YES 166 166 332
  NO 166 166 332
  TOTAL 332 332 664
  1. 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.