September 10, 2014

Cannabis graduation exaggeration


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.]

(I could also be picky and point out 3News have the journal wrong: The Lancet Psychiatry, which started this year, is not the same as The Lancet, founded in 1823)


Thomas Lumley (@tslumley) is Professor of Biostatistics at the University of Auckland. His research interests include semiparametric models, survey sampling, statistical computing, foundations of statistics, and whatever methodological problems his medical collaborators come up with. He also blogs at Biased and Inefficient See all posts by Thomas Lumley »


  • avatar
    Megan Pledger

    I agree with Prof Nutt. It’s hard to know if the mj use is a precursor or result of poor academic achievement (ditto – mental health problems). Poor academic achievement is signaled pretty early on.

    As for American standardised test data – they’ve been stuffing around with curriculum, tests and delivery modes that I don’t think there is going to be anything remotely consistent.

    3 years ago

  • avatar
    Darren Dahly

    This study was all over the radio yesterday. Glad to see others pointing out such obvious flaws in how it’s been presented. I hate these kinds of conclusions: the marijauana is the problem, so please just ignore the circumstances that result in a kid doing drugs *everyday*.

    3 years ago

  • avatar
    Frank Collette

    Due to the political forces on both sides of the weed argument, truth is nearly always the casualty.
    Both sides reinforce their arguments by either plain dishonesty, or as shown here [thank you] stretching truths, facts and figures.
    From my own thesis like exposure to the cannabis community, pure intelligence may actually be improved by cannabis use!!!! Schooling may not…..
    Real major problems are cardiovascular and endocrine disruption in susceptible peeps. Relationship problems in overuse and other varied but lesser problems.
    Smoking is the real culprit not usually the consumption of cannabinoids.
    No medical cannabis policy should embrace the act of smoking this herb. CO2 and CO3 plus numerous terpines etc are not beneficial for anyone.
    My herbalist has plant flowers steeped in alcohol, her extracted plant oils are administered with 5ml of 99% alcohol.
    The olde world thousand years old tradition of making hash and discarding 98% of the buds, allows users to avoid all the nasties normally ingested with smoking buds.

    If the government was serious about harm reduction, a methadone-like program in chemists, with users picking up 3-7 day supply of liquid tincture to place on the tongue.

    Many users are undiagnosed high functioning aspies and ADHDers who are self medicating their hidden and unaware condition.

    3 years ago

  • avatar

    Yes, unfortunately the Science Media Centre also fell foul of the Lancet Psychiatry press release in writing up the story – despite your most most helpful commentary. We have corrected our website.

    3 years ago

  • avatar
    Amy Linford

    I have just begun looking at this for one of my Assignments and I had many a question about the results. The exclusion of some important considerations made me question the depth of the research. Furthermore, the news articles, from where this topic post originated, failed to make it easy to access the initial study.
    Some of the numbers were rather strange!

    3 years ago