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Why power twomeans in Stata Does Not Always Need “Real Means”

Introduction A lot of people get stuck the first time they see this in Stata: power twomeans m1 m2, ... The syntax says means , so it feels natural to think m1 and m2 must always be the actual mean outcome in two groups . That is true in many ordinary superiority studies. But it is not the whole story. In practice, researchers sometimes enter values like 0 and 2 in a non-inferiority design and still get a valid sample size calculation. At first glance, that looks wrong. Why would “0” and “2”...