Noninferiority trial designs for odds ratios and risk differences

This study presents constrained maximum likelihood derivations of the design parameters of noninferiority trials for binary outcomes with the margin defined on the odds ratio (psi) or risk-difference (delta) scale. The derivations show that, for trials in which the group-specific response rates are equal under the point-alternative hypothesis, the common response rate, pi(N), is a fixed design parameter whose value lies between the control and experimental rates hypothesized at the point-null, {pi(C),pi(E)}. We show that setting pi(N) equal to the value of pi(C) that holds under H-0 underestimates the overall sample size requirement. Given {pi(C),psi} or {pi(C),delta} and the type I and II error rates, or algorithm finds clinically meaningful design values of pi(N), and the corresponding minimum asymptotic sample size, N = n(E) + n(C), and optimal allocation ratio, gamma = n(E)/n(C). We find that optimal allocations are increasingly imbalanced as psi increases, with gamma(psi) < 1 and gamma(delta) approximate to 1/gamma(psi), and that ranges of allocation ratios map to the minimum sample size. The latter characteristic allows trialists to consider trade-offs between optimal allocation at a smaller N and a preferred allocation at a larger N. For designs with relatively large margins (e.g. psi > 2.5), trial results that are presented on both scales will differ in power, with more power lost if the study is designed on the risk-difference scale and reported on the odds ratio scale than vice versa. Copyright (C) 2010 John Wiley & Sons, Ltd.