The problem with economists is that they have no way conduct scientific experiment on monetary policies. Yes, there are data, but interpretation is everything.
Economists can have interpretations that fit the data but it turns out to be bollock.
Yes, Economists may not like to hear this, but Economics is at best a proto-science. Beside the practical difficulty of performing controlled experiments with economic policies (as you pointed out), there's also the problem that Economics is not yet grounded on any meaningful substrate in the "great stack of science". Unlike Chemistry, for instance, which is firmly grounded on the Physics substrate.
Nevertheless, my opinion is that when eventually Economics does become a science it is more likely to resemble Keynesianism than the Austrian school so favoured by this forum's hivemind.
Ah, no. Economics is, indeed, a science. It's just that it's not that kind of science. It's a social science, not a physical science. As such, it's laws are based on how humans act, and therefore how they think. Hence the name of the Austrian classic,
Human Action. Anyone who trys to tell you that mathmatical formulas can be derived from the economic data, or that the laws of economics drive the public and not the other way around, (*cough* Keynes *cough*) is full of crap. Economics is the "dismal science" because it's a social science. Physics is easy, your experiments (above quantum) are not affected by observations. Humans don't act that way, they respond differently to the same stimuli, and moreso when they are aware that they are the experiment.