In Example 4.15 (p. 130) a statistically significant relationship was found between hormone therapy and invasive breast cancer. There were 166 cases of invasive breast cancer out of 8506 women taking hormones and 124 cases of invasive breast cancer out of 8102 women taking placebo.

a. What is the relative risk of invasive breast cancer when taking hormones compared with taking placebo?

b. What is the baseline risk of invasive breast cancer, assuming that the placebo group is representative of the relevant population of women?

c. Explain why it is important to report the baseline risk found in part (b) when reporting the relative risk found in part (a).

Example
Breast Cancer Risk Stops Hormone Replacement Therapy Study On July 17, 2002, the Journal of the American Medical Association published the results of a study that affected the lives of millions of women. The study was the first large randomized experiment to test the effects of combined estrogen and progestin, the most commonly prescribed hormone replacement therapy for postmenopausal women.