Measured how?
Unless there are secret spy cameras in everybody's home, how are you going to know how many people are actually smoking pot? If the study used any sort of polls or surveys, the results are meaningless, since it is quite possible that legalization of pot would double the percentage of people who would admit to using it, whether or not it had any effect whatsoever on the number of people who actually did.
Further, with just about any drug, usage will cause more problems for some people than for others. If the drug is being banned because it causes serious problems for some people, the n for the ban to accomplish a useful goal it should reduce usage among the people for whom the drug would otherwise cause serious problems. If the people among whom the ban reduces usage are those who would not have had problems anyway, what's the point?
But hey, thanks for playing on this thread. We've got some nice parting gifts for you backstage. Don't let the door hit ya.