Question: Several large observational studies suggested that women who take hormones such as estrogen after menopause have lower risk of a heart attack than women who do not take hormones. Hormone replacement became popular. But in 2002, several careful experiments showed that hormone replacement does not reduce heart attacks. The National Institutes of Health, after re-viewing the evidence concluded that the observational studies were wrong. Taking hormones after menopause quickly fell out of favor

(a) Explain the difference between an observational study and an experiment to compare women who do and don't take hormones after menopause.

(b) Suggest some characteristics of women who choose to take hormone that might affect the rate of heart attacks. In an observational study. the characteristics are confounded wok taking hormones.