Size of the treatment effect
PPH
EVENT
Based on the existing evidence in PPH prevention and surgical trials, we assumed a 25% reduction in PPH with tranexamic acid (RR = 0.75).A trial of about 30,000 women has almost 90% power to detect that (1) tranexamic acid is more effective than placebo and (2) IM tranexamic acid is at least 60% as effective as IV tranexamic acid. We assumed a two-sided type 1 error rate of 5% for the superiority arm and a one-sided rate of 2.5% for the non-inferiority arm. Our sample size and power calculations were made using methods described by Stucke and Kieser (2012) [Because the true event rate and treatment effect are unknown, we will re-estimate the sample size if unplanned interim analyses indicate a lower-than-expected event rate or if new information emerges which changes beliefs about the expected treatment effect.