Scale Unit variance Proportion (π)     0.05 0.1 0.15 0.2 0.3 0.4 0.5 Proportion, π (Risk difference) π(1 −π) 0.048 0.090 0.13 0.16 0.21 0.24 0.25 Logit: log(π/(1 − π)) (Odds ratio) [π(1 − π)]−1 22 12 7.9 6.3 4.8 4.2 4.0 Log: log(π) (Risk or rate ratio) (1 − π)/π 19 9.0 5.7 4.0 2.4 1.5 1.0

Table 2: Sample size multiplier (unit variance) when studying a difference or ratio of two proportions, or the ratio of their odds. Multipliers are shown as a function of the scale involved and of the value of response proportion (π), rounded up, and to two significant digits. As in generalized linear models, the multiplier for the logit and log of the proportion were derived by multiplying the unit Bernoulli variance under the identity link function by the Jacobian associated with the change of scale.