And the studies also influence medical practice -- doctors, the critics note, encouraged women to take hormones after menopause long before their effects were tested in randomized clinical trials, the gold standard of medical research.
Some of epidemiology's critics are calling for stricter standards before such studies get reported in medical journals or in the popular press.
Young, one of the foremost critics, argues that epidemiological studies are so often wrong that they are coming close to being worthless. "We spend a lot of money and we could make claims just as valid as a random number generator," he says.
Epidemiology's defenders say such criticisms are hugely overblown.
They are "quite simplistic and exaggerated," says Dr. Meir Stampfer, a professor of epidemiology and nutrition at the Harvard School of Public Health and a professor of medicine at Harvard Medical School.
What's more, some things simply cannot be tested in randomized clinical trials. In certain cases, to do so would be unethical. (Care to assign half the people in a trial to smoke cigarettes?)
In other cases, a trial of adequate size and duration -- say, to test whether coffee drinking raises or lowers the risk of Parkinson's disease -- would have to control the habits of huge numbers of people for decades. That would not only be hugely expensive but also virtually impossible.
Stampfer cites examples of findings of epidemiology that, he says, have stood the test of time: smoking's link to lung cancer, to name the most notable.
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Watching for clues
In epidemiological studies (also called observational studies), scientists observe what's going on -- they don't try to change it. From what they observe, they reach conclusions -- for example, about the risk of developing a certain disease from being exposed to something in the environment, lifestyle or a health intervention.
There are different ways to do this. Cohort studies follow a healthy group of people (with different intakes of, say, coffee) over time and look at who gets a disease. They're considered the strongest type of epidemiological study.
Case-control or retrospective studies examine people with and without a certain disease and compare their prior life -- for how much coffee they drank, for example -- and see if people who got the disease drank more coffee in their past than those who didn't.
