The limitations of nonexperimental research were dramatically brought to the attention of the public by the results of an experiment on the effects of postmenopausal hormone replacement therapy (part of a larger study known as the Women's Health Initiative). An experiment is called a clinical trial in medical research.

In the clinical trial, participants were randomly assigned to receive either the hormone replacement therapy or a placebo (no hormones). The hormone replacement therapy consisted of estrogen plus progestin.

In 2002, the investigators concluded that women taking the hormone replacement therapy had a higher incidence of heart disease than did women in the placebo (no hormone) condition. At that point, they stopped the experiment and informed both the participants and the public that they should talk with their physicians about the advisability of this therapy.

The finding dramatically contrasted with the results of nonexperimental research in which women taking hormones had a lower incidence of heart disease; in these studies, researchers compared women who were already taking the hormones with women not taking hormones. Why do you think the results were diff erent with the experimental research and the nonexperimental research?