Teenagers who use cannabis daily are seven times more likely to attempt suicide and 60 percent less likely to complete high school than those who don’t, latest research shows.
Me (via Science Media Center)
“The associations in the paper are summarised by estimated odds ratios comparing non-users to those who used cannabis daily. This can easily be misleading to non-specialists in two ways. Firstly, nearly all the statistical evidence comes from the roughly 1000 participants who used cannabis less than daily, not the roughly 50 daily users — the estimates for daily users are an extrapolation.
“Secondly, odds ratios are hard to interpret. For example, the odds ratio of 0.37 for high-school graduation could easily be misinterpreted as a 0.37 times lower rate of graduation in very heavy cannabis users. In fact, if the overall graduation rate matched the New Zealand rate of 75%, the rate in very heavy cannabis users would be 53%, and the rate in those who used cannabis more than monthly but less than weekly would be 65%.
That is, the estimated rate of completing high school is not 60% lower, it’s about 20% lower. This is before you worry about the extrapolation from moderate to heavy users and the causality question. The 60% figure is unambiguously wrong. It isn’t even what the paper claims. It’s an easy mistake to make, though the researchers should have done more to prevent it, and that’s why it was part of my comments last week.
You can read all the Science Media Centre commentary here.
[Update: The erroneous '60% less likely to complete high school' statement is in the journal press release. That's unprofessional at best.]