“In our need to understand, to explain, and to treat, the temptation to impute causality to association is pervasive and hard to resist. It is the most important reason for error in medicine.”
— Petr Skrabanek and James McCormick
, Authors of Follies and Fallacies in Medicine
Large and small percentages
Framingham is a small town near Boston, Massachusetts. Since the early-1950s a large number of Framingham citizens have taken part in a study surveying all factors that may play a role in the development of atherosclerosis and heart disease. Among other things their cholesterol was measured frequently.
After five years the researchers made an observation, which should become one of the cornerstones in the cholesterol issue. When they classified the citizens into three groups with low, medium and high cholesterol values they saw that in the latter group more had died from heart attacks than in the two other groups. A high cholesterol level predicted a greater risk of a heart attack, they said; high cholesterol is a risk factor for coronary heart disease.
The predictive value of blood cholesterol levels was confirmed in the greatest medical experiment in history, the Multiple Risk Factor Intervention Trial, also called MR.FIT. In that trial researchers measured the blood cholesterol of more than 300.000 American middle-aged men.
Six years later the director of MR.FIT, professor Jeremiah Stamler and his coworkers from Chicago asked how many of these men had died and from what. The participants were then divided into ten groups of equal size, so-called deciles, according to their cholesterol values. The first decile thus consisted of the tenth of the men with the lowest cholesterol, the tenth decile of the tenth with the highest cholesterol.
The researchers analysis showed that in the tenth decile four times more men had died of a heart attack than in the first decile. Professor Stamler’s team put it in another way: “the risk of dying from a heart attack with cholesterol above 265 mg/dl (6.8 mmol/l) was 413 percent greater than with cholesterol below 170.”
With statistics you can change black to white, or vice versa; as any politician will tell you. Four hundred and thirteen percent! A frightening figure.
But let us look at the real figures and not only at the percentages. How many men had, in fact, died from a heart attack?
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