This actually isn't well known. It's called the placebo effect. In medical studies, you have a control group (not given any medicine), the test group (getting the medicine), and the placebo group (they get a sugar pill but they're told it's the medicine from the test group).

What is found in nearly every study is that there will be a difference between the control group and the test group, but there will also be a difference between the control group and the placebo group. The placebo, for certain people, does "something." It may not be doing everything the test medication is doing, but it's not doing nothing. It's a psychological effect. Sometimes, it hasn't actually cured anything or fixed anything, but the person taking it feels like it has.

So in order for the medicine in the test to be considered to be working, it has to outperform the placebo group by a certain amount. This is what measurement you get about what the drug is actually doing.

Just to note: the placebo effect works even if you tell people it's a placebo. That's the wacky thing.