mv = pq is very well tested. MV = PQ is true by definition. There is nothing to test about it because it's tautologically true. Then again, people go astray a

mv = pq is very well tested. MV = PQ is true by definition. There is nothing to test about it because it's tautologically true. Then again, people go astray all the time when they try to fill the equation with causal implications which can be tested. For example, when people said "Increasing M will increase P" and predicted hyperinflationary mayhem as a consequence of quantitative easing. Those predictions have failed entirely, mostly because rather than change P or Q, the change in M simply changed V instead. (There is a plausible argument that quantitative easing has contributed to asset price inflation, but that doesn't have anything to do with MV = PQ either, because the P in that equation isn't about asset prices.) Because of these failures, the value of MV = PQ for economics as a science is basically zero.