I try my best to know what I don’t understand, and one of the things I know is that meta analyses and evidence reviews are difficult for lay people to assess. The merging of data from multiple sources is complex and understanding what’s going on requires actual knowledge of the character of the studies incorporated in the review.
So when the people from Say Nope to Dope last week trumpeted a new meta analysis of findings about cannabis and violence, I was cautious. In the case of this new paper from the University of Montreal, I didn’t have journal access anyway. But one of the headline findings struck me as extraordinary: the headline odds ratio for lifetime use was 1.94; meaning that anyone who has ever tried cannabis was nearly twice as likely to have committed a violent offence as as anyone who hadn’t. In a country like New Zealand, where 80% of us use cannabis at some point, this seemed to have some remarkable implications.
So I asked Professor Joe Boden, director of the Christchurch Health and Development Study, whose research interests include both substance use and “the social and psychological determinants of maladaptive behaviour including aggression and violence.”
He didn’t take long to come back: the Canadian paper had misquoted an odds ratio from a paper by the Dunedin Multidisciplinary Health and Development Study (the companion longitudinal study to the Christchurch one) “as being 6.9, when the adjusted OR is in fact 3.15.”
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