I tried to make this point in a recent column (it was about fiscal rather than monetary policy, but the same point applies), but I think Barry Ritholtz makes the point better and more succinctly:
Understanding Why You Think QE Didn't Work, by Barry Ritholtz: Maybe you have heard a line that goes something like this: The weak recovery is proof that the Federal Reserve’s program of asset purchases, otherwise known as quantitative easement, doesn't work.
If you were the one saying those words, you don't understand the counterfactual. ...
This flawed analytical paradigm has many manifestations, and not just in the investing world. They all rely on the same equation: If you do X, and there is no measurable change, X is therefore ineffective.
The problem with this “non-result result” is what would have occurred otherwise. Might “no change” be an improvement from what otherwise would have happened? No change, last time I checked, is better than a free-fall.
If you are testing a new medication to reduce tumors, you want to see what happened to the group that didn't get the test therapy. Maybe this control group experienced rapid tumor growth. Hence, a result where there is no increase in tumor mass in the group receiving the therapy would be considered a very positive outcome.
We run into the same issue with QE. ... Without that control group, we simply don't know. ...