The effect of using postmenopausal hormones (PMH) on health outcomes is controversial. Most previous data col- lected have been from observational studies, and users of PMH may selectively differ from nonusers in ways that are difficult to quantify (e. g., more health conscious, more physi- cian visits in which disease outcomes can be identified). A clinical trial was planned to randomize postmenopausal women to either PMH use or no PMH use and follow them for disease outcomes over a 10-year period. One outcome of special interest was breast cancer. Suppose the incidence rate of breast cancer among post- menopausal 50-year-old women who do not use PMH is 200 per 10^5 women per year. Suppose also it is hypoth- esized that PMH increases the incidence of breast cancer by 20%. How many women need to be studied in each group (equal sample size per group) to have an 80% probability of detecting a significant difference if a two-sided test is used with a type I error of 5%