In his latest Economic View column, Richard Thaler uses a bit of probability theory to explain why the recent federal recommendation on delaying breast cancer screenings until age 50 is not such a terrible policy prescription.
Here is a quiz: Suppose that there is a one-in-1,000 chance that a woman in her 40s with no symptoms has breast cancer, and that 90 percent of the time a mammogram correctly classifies women as having cancer or not. If a woman in this group tests positive on her mammogram, what is the chance that she has cancer? The answer is not 90 percent. It is less than 1 percent, because of the large number of false positive results.
Thankfully, breast cancer is rare enough in this age group that a vast majority of positive hits are false positives. Do patients understand this while they are waiting for the results of the subsequent tests?
Full column is here.