Researchers at the University of Pennsylvania…

Researchers at the University of Pennsylvania have now issued a correction to their original report that teen marijuana use had no effect on the development of a psychotic disorder by age 36: http://www.independent.co.uk/life-style/health-and-families/health-news/marijuana-users-no-more-likely-to-experience-depression-psychosis-or-asthma-study-says-10443195.html., reveals that in fact their data showed a trend towards a 2.5-fold greater prevalence of psychotic disorders in the marijuana users, a trend which reached significance in a “one-tailed” statistical test.  Such a test is generally deemed appropriate if abundant prior studies have shown the same degree and direction of effect, as is the case here.  In their original report, they inappropriately corrected for demographic factors that overpowered the marijuana effect.  For example, they corrected for current socioeconomic status, when such status is an outcome of psychosis itself.  They also corrected for whether or not the individuals had health insurance at age 36; again, this is a factor that is most likely to be a consequence of psychosis rather than a cause, because psychotic individuals found it very difficult to maintain jobs and the attendant health insurance policies in the pre-Obamacare era.  Also note that the 2.5-fold increased risk of psychosis pertained to the low-strength marijuana more common in the 1980’s when the study began, whereas a 5-fold increase in risk is reported for the current high strength varieties: http://www.west-info.eu/skunk-increases-the-risk-of-psychosis/14tlp0454_di-forti/
 
The full length version of their published correction, which can be purchased for ~$11 online