Question: Refer to a study published in the Journal of the American Medical Association and reported in the Sacramento Bee on January 26, 2000 ("Study Ties Hormone Replacement, Cancer" by Shari Roan, p. A1). A sample of 46,355 postmenopausal women were studied for 15 years and asked about their use of hormone therapy and whether or not they had breast cancer; 2082 developed the disease. The article reported that "for each year of combined [estrogen and progestin] therapy, a woman's risk of breast cancer was found to increase by 8 percent compared to a 1 percent increase in women taking estrogen alone."

Assuming that the women and their physicians made the decision about which treatment to pursue (combined hormones, estrogen alone, or no hormones), discuss the possibility of confounding variables in this study.