Presenting Health Statistics
By Arnold Kling
A 31 percent reduction in heart attacks, after all, seems impressive. Yet this pervasive way of describing clinical trials in medical journals—focusing on the “relative risk,” in this case of heart attack—powerfully exaggerates the benefits of drugs and other invasive therapies…
There’s another instructive way to consider the numbers. Suppose that 100 people with high cholesterol levels took statins. Of them, 93 wouldn’t have had heart attacks anyway. Five people have heart attacks despite taking Pravachol. Only the remaining two out of the original 100 avoided a heart attack by taking the daily pills. In the end, 100 people needed to be treated to avoid two heart attacks during the study period—so, the number of people who must get the treatment for a single person to benefit is 50. This is known as the “number needed to treat.”
Fair enough. But at least with clinical trials, we can calculate the number needed to treat (NNT), even if it is not the number that gets reported. I respectfully submit that many medical protocols, including referrals for diagnostic imaging and for seeing a specialist, have much worse NNT’s, but nobody is tracking those numbers. I’ll bet that the number of people you need to send for an MRI when they have back pain in order for a single person to benefit is in the hundreds, if not the thousands. I’ll bet that the number of people with microscopic hematuria that you need to send for cystoscopy and IVP in order for a single person to benefit is also quite high.
By the way, Nortin Hadler, in his book The Last Well Person, is also not happy with the relative risk measure of effectiveness.
Thanks to Trent McBride (email) for the pointer).