A study published by the American Journal of Epidemiology last month found that 12.2 percent of drivers killed by car crashes in six states tested positive for cannabinol, a marijuana metabolite, in 2010, up from 4.2 percent in 1999. Here is how NBC News translated that finding in the headline over a story posted on Saturday: “Pot Fuels Surge in Drugged Driving Deaths.” The article, which begins by describing the deaths of a Colorado woman and her infant son in a crash caused by “a driver who admitted he smoked pot that day,” links the purported surge in marijuana-related traffic fatalities to laws allowing medical use of cannabis. “As medical marijuana sales expanded into 20 states,” writes health reporter Bill Briggs, “legal weed was detected in the bodies of dead drivers three times more often during 2010 when compared to those who died behind the wheel in 1999.” There are several problems with reading the trend described by this study as evidence that legalizing medical marijuana causes an increase in fatal car crashes: