Don’t drink and drive, smoke dope and fly.

Stuff is reporting on a new paper in the BMJ (or as Stuff calls it, only 24 years after the name change, the British Medical Journal) saying that smoking dope doubles your chance of  a serious or fatal car accident.  Regular readers of StatsChat posts on drug journalism will be pleasantly surprised to hear that this is actually what the research paper says, and that the researchers had moderately good evidence for this conclusion.

The paper compiled the results of previous studies, restricting their attention to studies that had a control group, and that measured  actual THC concentration in the blood, rather than the inactive metabolites that can be detected days later in blood or urine.   Some of the studies compared drivers in crashes to drivers not in crashes, others compared the driver responsible for the crash to other drivers involved in the crash.

The summary of the findings looking at various ways of breaking up the data is:

That’s a forest plot, with the dots showing the estimated risk ratio [technically odds ratio, but it doesn’t matter in this context] and the lines showing the 95% margin of uncertainty.   Though some of the lines include the null value 1.0 and others don’t, there’s pretty clear agreement across the analyses that cannabis increases crash risk, by a factor of two or so.

After getting it right so far, Stuff unfortunately went further and said “The research found cannabis significantly impaired the psychomotor response, or muscle activity linked to mental processes.”  That isn’t actually what they found. That’s in the Introduction section of the paper, because it’s telling us what was already known — dope makes you slower and clumsier.  The BMJ isn’t publishing research telling us that, any more than they publish conclusions on the spatial distribution of ursine excretory activities, or the religious affiliation of the Pope.

They finish up by saying “The Land Transport Amendment Act 2009 will be reviewed this year and police are investigating the possibility of introducing more effective roadside drug tests.” This paper doesn’t really lend much support to changes in current roadside drug tests, which have the advantage of testing for actual impairment and not being specific to a single drug.