Of 18 total studies, 10 met all of the criteria. All studies reported an increased risk for psychosis-like events or schizophrenia, and in nine of the ten studies there was a significant increase. Overall, the odds ratio of developing psychosis like events or schizophrenia was 2.88. If the cannabis use occurred during adolescence, the risk was two times higher. The Hill criteria indicated a high likelihood for the contribution of cannabis to schizophrenia development. (The Bradford Hill criteria are used in epidemiology to evaluate the strength of evidence for a causal relationship between an exposure and an outcome.)