What kind of study was it? You have to really look at the methodologies of these things. There are plenty of studies that show a correlation between schizophrenia and cannabis use, but correlation proves nothing. It could just be that schizophrenics are more likely to meet stoners, or that schizophrenics have more reasons to smoke weed, or that some other factor makes you more likely to become schizophrenic and more likely to smoke weed.

It's like the study that proved a correlation between crime rates and ice cream sales, just to make a point. What was that point? It's not that buying ice cream makes you more likely to commit crimes, but that hot weather makes more people buy ice cream and makes more people commit crimes.