You mean where's the evidence that evidence based medicine works?

I could produce some evidence but I'm afraid that then you'd ask me where's the evidence that my evidence actually supports the hypothesis that evidence based medicine actually helps.

That's the problem of induction, a.k.a. "how do we know the sun will rise tomorrow?" problem.

http://en.wikipedia.org/wiki/Problem_of_induction

Lots of old people. Though that might just be down to the decrease in witch burnings as a method of driving out plague. Hard to tell for sure unless we count them, but there does seem to be loads of them about.