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Saturday 5th May 2012

There’s an interesting discussion about peer group substance use in the latest edition of ‘Addiction’^1,2. It set me thinking about what little we really know in spite of all the time spent on research.

The notion is that the perception of peer group substance use can influence an individual’s own decision on whether and how much to use. It has been suggested, recently, that people tend to over-estimate the substance use of those around them and might, therefore, be more prone to use themselves.

The discussion in Addiction centred on whether the way the data is being collected provides useful information and if the data collection is flawed what is the direction of that flaw.

There are those who agree with the notion that peer group use is overestimated. Others are happy that the data provides accurate estimates and, yet others, who think that rather than peer group use being over-estimated, the individual is under-reporting his/her own use.

That, of course, is a key flaw in self-report surveys; is the subject telling the truth? There are ways in which surveying organisations try and encourage truth-telling and identify lies but I’m not convinced that these are effective. The British Crime Survey asks people about substance use and includes a fictitious substance on the basis that this will identify people who don’t know what they are talking about when it comes to psychoactive substances so that their responses can be ignored. But it seems just as likely, especially with younger people, that someone who is very knowledgeable about psychoactives would report use of the fictitious drug intentionally to distort the survey’s findings.

As well as participants who might knowingly try and provide false information there is the risk of people reporting what they think they should say. I think it is fair to assume that if you asked people how many portions of fruit and veg they eat each day the responses would cluster around the five that is endlessly promoted as the healthy guideline.

Added to the difficulty of collecting reasonably accurate data from self-report surveys is the way in which the surveying organisations make use of that data.

Two recent reports from the USA serve to illustrate some of these issues.

Monitoring the Future (MTF)³ is a long-running survey undertaken by the University of Michigan amongst children from the 8th, 10th and 12th grades in US schools. The Partnership Attitude Tracking Study (PATS)⁴ is a survey conducted amongst children in the 9th to 12th grade by The Partnership at Drugfree.org

Interestingly, the PATS report seems to have attracted more attention than the MTF. That is surprising because the MTF is, by far, the better survey. As well as having a much longer history of conducting surveys, the MTF uses a much higher number of respondents; PATS has a sample size of 3,322 against MTF’s 46,700 in total, with 30,300 of those in the same age range as used by PATS. Size isn’t everything, of course, but the larger a survey is the more likely it is to get representative results.

The other important thing about surveys is how they are presented. The Partnership at Drugfree.org began life as the Media-Advertising Partnership for a Drug-Free America and that tells us two important things about its viewpoint.

The headline on the page giving access to the PATS report says;

‘National Study: Teen “Heavy” Marijuana Use Up 80 Percent Since 2008, One in Ten Teens Reports Using Marijuana at Least 20 Times a Month’⁴

I’ll return to that claimed 80% increase in a moment but first it is worth noting that the figures in the PATS survey show 9% of teens using marijuana at least twenty times a month. ‘One in ten’ is 10% and the difference between 9% and 10% is 11% so the headline is substantially over-stating the claimed prevalence.

Let’s return to that 80% rise in ‘heavy’ marijuana use. It is not completely clear what is meant by ‘using marijuana at least 20 times a month’. The MTF survey looks at daily use and finds that to be 6.6% in the matching age group rather than 9%. However, I think that what PATS means by using 20 times a month may be having 20 joints not using on 20 days.

The comments under the PATS announcement⁴ include one from ‘the researcher’ who says ‘teens said they smoked 20X or more per month’. That suggests to me that each joint is being counted and would explain why PATS comes up with a prevalence that is 50% higher than MTF’s daily prevalence.

It’s important not to lose track of what these percentages mean in real numbers. For the MTF survey, the recorded increase in daily cannabis use of 26% since 2008 equates to 90 teenagers from the survey group of 30,300.

The narrative accompanying the PATS report is all about the notion that talking about medical marijuana is making it more acceptable to young people. The people behind The Partnership at Drugfree.org are, of course, opposed to any use of cannabis so their views are from that perspective. It is no wonder that drug warriors like Kevin Sabet have leapt onto the headline results with glee.

One of the other comments echoes a point that had occurred to me. The MTF survey looks at alcohol as well as the illegal substances and whilst it finds a 26% increase in daily cannabis use since 2008 it also records a 17% decline in daily alcohol use. It is not in the nature of things that you would be able to find any individual who switched from daily alcohol to daily marijuana but, if such an individual existed, he/she would have greatly increased life expectancy as a result.

It is true that there has been more truthful information about the short and long-term effects of cannabis demonstrating that it is far less harmful than its classifications, both in the USA and the UK, would have you believe. At the same time, there has been greater focus on the serious harms often caused by problem alcohol use. Given that both PATS and MTF are self-report surveys, it is possible that participants believe they are reporting what is expected rather than their actual behaviour.

References

^1.Young people's overestimation of peer substance use: an exaggerated phenomenon? Addiction Volume 107, Issue 5, pages 878–884, May 2012
^2.Commentaries on Pape Addiction Volume 107, Issue 5, pages 885–891, May 2012
^3.Johnston, L. D., O’Malley, P. M., Bachman, J. G., & Schulenberg, J. E. (2012). Monitoring the Future national results on adolescent drug use: Overview of key findings, 2011. Ann Arbor: Institute for Social Research, The University of Michigan.
^4.The Partnership Attitude Tracking Study 2nd May 2012 The Partnership at Drugfree.org

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