Cross-sectional studies compare people's present lifestyle (how much coffee they drink now) with their present health status.
Epidemiological studies have several advantages: They are relatively inexpensive, and they can ethically be done for exposures to factors such as alcohol that are considered harmful, because the people under study chose their exposure themselves.
But epidemiological studies have their minuses too, some of which are very well known. Suppose a study finds that coffee drinkers are more likely to get a certain disease. That doesn't mean coffee caused the disease. Other, perhaps unknown, factors (called "confounders" in the trade) that are unrelated to the coffee may cause it -- and if coffee drinkers are more likely to do this other thing, coffee may appear, incorrectly, to be the smoking gun.
A much clearer picture of the role of coffee on disease could be found, in theory, via a randomized clinical trial. One would divide a population into two, put one group on coffee and the other not, then follow both groups for years or decades to see which group got certain diseases and which didn't.
The problem, however, is that such a study is very expensive and takes a long time, and it can be difficult to control people's lives for that length of time.
Despite their shortcomings, epidemiological studies are often taken seriously, so much so that they can change medical practice. Such was the case after dozens of epidemiological studies, including one large, frequently cited one that came out of Harvard in 1991, had shown that taking estrogen after menopause reduces the risk of women getting cardiovascular disease.
"There was such a belief," even with the medical community, that hormone replacement became part of standard medical practice, says Dr. Lisa Schwartz, associate professor of medicine at Dartmouth Medical School in Hanover, N.H., even in the face of an increased potential risk of breast cancer. In fact, some scientists and doctors said it would be unethical to do a randomized clinical trial to check if the hormone effect was real.
But in the hormone epidemiological studies, women choosing to take hormones may well have been healthier in other ways, Kramer says. And that fact -- that they were healthier -- could explain the lower risk of heart disease, not the hormones.
"To get hormone therapy, you have to go to a doctor and have to have insurance," Kramer says. "That means you are in the upper strata of society."
