Confounding
When the Nurses’ Health Study first published its observations on estrogen and heart disease in 1985, it showed that women taking estrogen therapy had only 1/3 the risk of having a heart attack as had women who had never taken it. Only 90 heart attacks had been reported among the 32,000 postmenopausal nurses in the study.
In 1987, Diana Petitti reported that she, too, had detected a reduced risk of heart-disease deaths among women taking HRT in the Walnut Creek Study, a population of 16,500 women. However, she “found an even more dramatic reduction in death from homicide, suicide and accidents.” “Healthy-user bias” appeared to be “confounding” the association.
People who faithfully engage in activities that are good for them are fundamentally different from those who don’t. Women who take HRT differ from those who don’t: they’re thinner; they have fewer risk factors for heart disease to begin with; they tend to be more educated and wealthier; to exercise more; and to be more health conscious.
In a large population studied by Elizabeth Barrett-Connor, having gone to college was associated with a 50% lower risk of heart disease.
According to George Davey Smith, the poor are less educated than the wealthy; they smoke more and weigh more; they’re more likely to have hypertension and other heart-disease risk factors, to eat what’s affordable rather than what the experts tell them is healthful, to have poor medical care and to live in environments with more pollutants, noise and stress.
People who comply with their doctors’ orders when given a prescription are different and healthier than people who don’t.
The Coronary Drug Project set out in the 1970s to test whether clofibrate might prevent heart attacks. The subjects were some 8,500 middle-aged men with established heart problems. Two-thirds of them were randomly assigned to take one of the 5 drugs and the other 1/3 a placebo.
After 5 years, those men who said they took more than 80% of the pills prescribed fared substantially better than those who didn’t. Only 15% of these faithful “adherers” died, compared with almost 25% of the “poor adherers.” This might have been taken as reason to believe that clofibrate actually did cut heart-disease deaths almost by 1/2, but then the researchers looked at those men who faithfully took their placebos. Only 15% of them died compared with 28% who were less conscientious.