If you get a test for something at the doctor and it comes back positive, chances are you're going to be very anxious. Does that mean you have the disease? Maybe. But as Aaron Carroll explains below, this can be a misleading way to think.
The prevalence of the disease — that is, the number of people in the general population who have it — can sharply effect one's chances with such a test. One study of mammograms, for example, found that over 95 percent of people with a positive test for breast cancer did not actually have it.
That's only the first part of a complex topic, but it's really worth understanding. Check it out. Ryan Cooper
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