“Why do medical tests always have error rates?”

John Cook writes:

Someone recently asked me why medical tests always have an error rate. It’s a good question.

A test is necessarily a proxy, a substitute for something else. You don’t run a test to determine whether someone has a gunshot wound: you just look.

Good point! I hadn’t thought about it that way before, but, yeah, “Let’s test for condition X” implies an indirectness of measurement, in a way that “Let’s look for X” does not.

Also, yeah, sure, sometimes looking isn’t enough to tell if someone has a gunshot wound; there have to be some edge cases. But we get Cook’s general point here.