Right, as I expected, any amount of uncertainty and you'll cling to it.
"“People who reported new psychotic symptoms, and who persisted in using cannabis ... reported more persistent psychotic symptoms than those who stopped using cannabis,” write Wayne Hall, PhD, of the University of Queensland in Herston, Australia, and Louisa Degenhardt, PhD, of the Burnet Institute in Melbourne, Australia, in an accompanying editorial."
"“Carefully conducted prospective studies such as this one provide the best evidence available that cannabis contributes to the cause of some cases of schizophrenia,” says Matthew Large of Prince of Wales Hospital in New South Wales, Australia. Large recently published a similar study linking marijuana use to earlier age of onset of schizophrenia.
“The study goes a long way to clarifying that it is not psychosis that causes cannabis use, but rather the reverse,” he says in an email."
So the fact that people who smoke pot reported more persistent psychotic symptoms is not evidence, apparently. Let's be honest, there's nothing a study could tell you that you'd ever accept as evidence that marijuana has negative effects.