Austin Frakt and Aaron E. Carroll in The New York Times:
Consider aspirin for heart attack prevention. Based upon both modifiable risk factors like cholesterol level and smoking, and factors that are beyond one’s control, like family history and age, it is possible to calculate the chance that a person will have a first heart attack in the next 10 years. The American Heart Association recommends that people who have more than a 10 percent chance take a daily aspirin to avoid that heart attack. How effective is aspirin for that aim? According to clinical trials, if about 2,000 people follow these guidelines over a two-year period, one additional first heart attack will be prevented. That doesn’t mean the 1,999 other people have heart attacks. The fact is, on average about 3.6 of them would have a first heart attack regardless of whether they took the aspirin. Even more important, 1,995.4 people would never have a heart attack whether or not they took aspirin. Only one person is actually affected by aspirin. If he takes it, the number of people who remain heart attack-free rises to 1996.4. If he doesn’t, the number remains 1995.4. But for 1,999 of the 2,000 people, aspirin doesn’t make any difference at all.
…The Mediterranean diet, which is heavy in vegetables, fruits, nuts and olive oil; moderate in fish and poultry; and light in dairy, meat and sweets; has long been advocated as a means to avoid heart disease. In people who have never had a heart attack, but who are at risk, the N.N.T. is 61 to avoid a heart attack, stroke or death. And that is for people who adhere to the diet for about five years. For those at higher risk, who have already had a heart attack, to avoid one additional death, the N.N.T. is about 30. That’s the number of people who would have to adhere to the diet for four years so that one extra person survived. About 1.4 people out of 30 such people will die no matter what they eat; 27.6 will not die no matter what they eat. Only one will benefit from sticking to the diet.