Still, there's a lot of room between "no evidence" and "double-blind clinical trials", and reason to believe that statements about software development can profitably be moved towards the latter.

I would take Code Complete as a good example. There are sections on methodology that run along the lines of "Company A tried this method and got a B% reduction in defects compared to the first version of the product. Company X also used it and only got a Y% improvement compared to the previous year, but that's still worth using." Studies don't have to be totally conclusive in order to provide meaningful back-up to anecdotal reports and subjective experience.

In other words, the perfect is the enemy of the good, and good evidentiary metrics may be achievable even if perfect ones aren't.