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Pontifications on Poison

Being some ramblings on events associated with poisonous plants.

Thursday 27th February 2014

This is one of those where I thought I could get my comment into the 140 characters Twitter allows but then realised I had more to say about it. I’m not a great fan of using multiple Tweets to tell a story so I’m here instead.

It concerns a paper from the International Journal of Drug Policy entitled  ‘Correlates of Intentions to Use Cannabis among US High School Seniors in the Case of Cannabis Legalization’. The abstract is here but I decided I should pay for the full paper in order to be sure that I was not being misled by poor abstracting.

It came to my attention because of this headline from UPI;

‘Surprise: high school seniors expected to smoke more pot now that it's legal’

That, of course, was leapt on by Kevin Sabet who Tweeted;

‘Researchers expect high school marijuana use to rise with legalization’

Hemp News gave a link to this slightly more measured article in Science Daily;

‘Prevalence of high school seniors' marijuana use is expected to increase with legalization’

Though we’ll see that the headline is deeply flawed.

The research was done by analysing the Monitoring the Future (MTF) survey, the USA’s main survey of substance use in young people.

The opening line of the ‘Results’ section of the abstract says;

Ten percent of non-cannabis-using students reported intent to initiate use if legal and this would constitute a 5.6% absolute increase in lifetime prevalence of cannabis use in this age group (emphasis added for reasons I’ll return to).

Once you get to the full paper, the authors explain that there is little data available on the effects of legalization on use but note that ‘cannabis decriminalization or depenalization have generally not been associated with increased rates of use’.

The MTF asks this question;

“If marijuana were legal to use and legally available, which of the following would you be most likely to do?”

The survey participants are given the options;

1) “Not use it, even if it were legal and available,”
2) “Try it,”
3) “Use it about as often as I do now,”
4) "Use it more often than I do now,”
5) “Use it less than I do now,”
6) “Don't know.”

The authors report that ‘10.3% indicated they would try cannabis if legal to use and legally available’.

And it is that figure that has been jumped on. I’ll try and explain why I think it is meaningless. The question asks about what happens if cannabis is legal but does not qualify that by defining who it is legal for. In Colorado, a state that has attracted all the attention by making cannabis legal for recreational users, use by under 21s remains illegal.

So the high school children who say they would not use cannabis because to do so would be breaking the law would, presumably, still say they would not use under Colorado’s regime. The ‘presumably’ is because, of course, no-one has asked that question.

The authors do a pretty thorough job of examining the limitations of their own study including pointing out that;

‘MTF did not specify under which (if any) regulatory conditions cannabis would be used (e.g., sold in stores, age restrictions),’

But they don’t seem to have considered the difference between intention to use when legal for the survey participants and intention to use when legal for older people only. I am sure they will be some young people who feel that the age limit is arbitrary and will be willing to break the law where previously they respected it but there is no-one who can begin to estimate that group because the question hasn’t been asked of them.

There is also no definition of what ‘legal’ means and that is important in the USA. The question that should be asked is ‘Would you use cannabis if legal in your state even though it remained illegal under federal law?’

There’s a final, and central, confusion that appears in the conclusions.

‘…it is unknown whether the same students who reported intention to use if legal are the same students who will initiate use during adulthood regardless of legal status.’

What that is pointing out is that the ‘10.3% [who] indicated they would try cannabis if legal to use and legally available’ has not been broken down into those who will wait until it is legal for them by age and those who will ignore the age restriction. That's what makes the 'high school seniors' part of the headline above wrong.

I’m not criticising the authors. The work they have done seems to have been done properly and reported thoroughly. What I am saying is that it doesn’t actually produce any useful results because the data from the MTF isn’t helpful when looking at the impact of changes in the regulation of cannabis.

The problem arises because these limitations in the data are ignored by the mainstream reporting of this work and the bald conclusion of the abstract;

‘Prevalence of cannabis use is expected to increase if cannabis is legal to use and legally available.’

is jumped on by those who, because the study is based on a survey of schoolchildren, go along with Science Daily’s misreporting in its headline ‘Prevalence of high school seniors' marijuana use is expected to increase with legalization’.


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