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Title: If Marijuana Causes Lots Of Crashes, Why Are They So Hard To Count?
Source:
Forbes
URL Source: http://www.forbes.com/sites/jacobsu ... why-are-they-so-hard-to-count/
Published: Feb 12, 2015
Author: Jacob Sullum
Post Date: 2015-06-21 15:05:18 by Deckard
Keywords: None
Views: 1234
Comments: 8
Last year, during a congressional hearing on the threat posed by stoned drivers, a representative of the National Highway Traffic Safety Administration (NHTSA) was asked how many crash fatalities are caused by marijuana each year. “That’s difficult to say,” replied Jeff Michael, NHTSA’s associate administrator for research and program development. “We don’t have a precise estimate.” The most he was willing to affirm was that the number is “probably not” zero. Michael knows something that grandstanding politicians and anti-pot activists either do not understand or refuse to acknowledge: Although experiments show that marijuana impairs driving ability, the effects are not nearly as dramatic as those seen with alcohol, and measuring the real-world consequences has proven very difficult, as demonstrated by a landmark study that NHTSA released last Friday. In “the first large-scale [crash risk] study in the United States to include drugs other than alcohol,” NHTSA found that, once the data were adjusted for confounding variables, cannabis consumption was not associated with an increased probability of getting into an accident. Some news outlets accurately reported that result, and some did not, apparently because some reporters actually read the study, while others were content to skim NHTSA’s press release. Such carelessness misleads policy makers who are grappling with the issue of how to determine when people are too stoned to drive. It also aids pot prohibitionists, who cite the prospect of more blood on the highways as an important reason to resist legalization. The NHTSA study included more than 3,000 drivers who were involved in crashes during a 20-month period in Virginia Beach, Virginia, plus 6,000 controls who drove in the same area during the same period but did not get into accidents. As usual, the study found that alcohol use was strongly correlated with crash risk. After adjustment for confounding, the crash risk for drivers with a blood-alcohol content (BAC) of 0.08 percent was twice the crash risk for sober drivers; it was six times as high for drivers with a BAC of 0.10 percent and 12 times as high at a BAC of 0.15 percent. But the picture for marijuana was quite different. Over all, drivers who tested positive for active THC were 25 percent more likely to be involved in crashes. But once the researchers took sex, age, and race/ethnicity into account, the risk ratio shrank from 1.25 to 1.05 and was no longer statistically significant: This analysis shows that the significant increased risk of crash involvement associated with THC and illegal drugs…is not found after adjusting for these demographic variables. This finding suggests that these demographic variables may have co-varied with drug use and accounted for most of the increased crash risk. For example, if the THC-positive drivers were predominantly young males, their apparent crash risk may have been related to age and gender rather than use of THC. Further adjusting for alcohol consumption made the crash risk of cannabis consumers equal to that of drivers who tested negative for alcohol and all other drugs. In other words, the analysis, which NHTSA described as “the most precisely controlled study of its kind yet conducted,” provides no evidence that marijuana use increases crash risk. That result, the authors note, is similar to what the best-designed previous studies have found: a small or nonexistent increase in crash risk. Several reporters understood this crucial point and communicated it to their readers. In a story headlined “Feds: No Link Between Pot and Car Crashes,” The Hill’s Jesse Byrnes reported that “marijuana use has not been found to increase the risk of car crashes, according to a new federal report.” Under the headline “U.S.: Pot Use Doesn’t Increase Crash Risk,” David Shepardson of The Detroit News reported that “a government study released late Friday found no evidence that marijuana use leads to a higher risk of getting into a traffic crash.” CBS News, Huffington Post reporter Matt Ferner, and Washington Post drug policy blogger Christopher Ingraham correctly noted the uncertainty about marijuana’s impact on highway safety, emphasizing that alcohol poses a much clearer and more serious risk. Continued here
Jeff Michael of NHTSA (Image: House Oversight and Government Reform Committee)
(Source: NHTSA)
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Begin Trace Mode for Comment # 7.
#1. To: Deckard (#0)
They did count them. But then they ignored them because they chose to blame the crash on age and gender rather than the fact the driver was high as a kite. Ta-da! No marijuana-related crashes!
Statistics is tricky and it is hard to separate factors and establish causality. Often A and B have high corelation because they have common C cause. The is only an example.
#8. To: A Pole (#7)
This was a study where A causes C and B causes C. But when A and B are involved together ... the researchers are going to ignore A and place the blame on B.
Top • Page Up • Full Thread • Page Down • Bottom/Latest"If Marijuana Causes Lots Of Crashes, Why Are They So Hard To Count?"
#7. To: misterwhite (#1)
Replies to Comment # 7. "Often A and B have high corelation because they have common C cause."
End Trace Mode for Comment # 7.
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