Risk difference (Δ)   0.05 0.10 0.15 0.20 0.25 0.30 0.35 0.40 0.45 Δ−2 400 100 45 25 16 11 8.2 6.3 4.9 Odds, risk, or event-rate ratio (eΔ)   1.1 1.25 1.5 1.75 2.0 2.5 3.0 4.0 5.0 Δ−2 : 110 20 6.1 3.2 2.1 1.2 0.83 0.52 0.39

Table 6: Sample size multiplier, Δ−2, when studying a difference or ratio of two proportions, or the ratio of their odds, or the ratio of two event-rates. Multiplier is given as a function of the differerence, Δ, in the scale in question, between the alternative and null values of the comparative parameter.