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Akkana's Musings on Open Source, Science, and Nature.

Sat, 31 Jul 2010

Bogus statistics on drug use among drivers

The "Roadshow" column in yesterday's Merc had some pretty ... odd ... statistics involving marijuana and driving.

It quotes "an NHTSA report" as saying:

contrary to popular belief, marijuana has been found to play a significant role in car accidents across the United States, with as much as 33 percent of drivers arrested at the scene of the accident being positive for marijuana and another 12 percent testing positive for marijuana and cocaine. Every year, 28 percent of drivers in the U.S. will attempt to drive within two hours after ingesting alcohol or illicit drugs. Marijuana is the drug used most often — 70 percent — by drivers who drove after drug use and is a major factor why crashes are the leading cause of death for American young people.

Whoa. Let's play that back again: 45 percent of all drivers arrested at accident scenes (33 plus another 12) test positive for marijuana? Nearly half?

Mr. Roadshow, you don't really believe that number, do you?

I didn't. So I did some searching, looking for the NTHSA source.

When I searched for large portions of the quoted phrase, I didn't find anything from the NHTSA. The Roadshow quote appears to come from an article on (I'm sure that's an unbiased source). Here's their MS Word file or Google's cached HTML version). The same article is also available as a PDF at and there are lots of other pages making reference to it.

The article cites "Brookoff, Cook & Mann, 1994; Sonderstrom, Dischinger, Kerns & Trillis, 1995." for the 33% number. There's no citation offered for the "28% will attempt to drive...". They credit "NHTSA, 2000" for "Marijuana is the drug used most often ... by drivers who drove after drug use", but that one's not important because it says nothing about prevalence in accidents, merely that it's used more often than other drugs (no surprise there).

The NHTSA weighs in

Googling on a more general set of terms, I found my way to a October 2000 NHTSA report, Field Test of On-Site Drug Detection Devices. It's a roundup of many different studies, with drug use numbers all over the map, though none larger than the 33% figure and certainly nothing near 45%. That 33% figure is near the bottom:

Brookoff et al. (1994) used on-site testing devices in a study that found a 58% prevalence rate for drugs in subjects arrested for reckless driving (who were not found to be impaired by alcohol). The Brookoff team found that 33% of their sample tested positive for marijuana, 13% for cocaine, or 12% for both. (Because of sampling flaws in the study, these drug test rates should not be interpreted as drug prevalence rates for reckless drivers.) Interestingly, the on-site device (Microline) used by Brookoff and his colleagues generated a significant false positive rate for marijuana when compared to GC/MS results.

The horse's mouth

So what about the original study? I wasn't able to find Dischinger, Kerns & Trillis, but here's Brookoff et al. at the New England Journal of Medicine: Testing Reckless Drivers for Cocaine and Marijuana (cookies required).

A couple of important notes on the study: the figures represent percentage of drivers arrested for "reckless driving that would constitute probable cause to suspect intoxication by drugs", who were not considered to be under the influence of alcohol, and who were suspected of being under the influence of marijuana or cocaine ("all patrol officers were told that they could summon [the testing van] if they stopped a person suspected of driving recklessly under the influence of cocaine or marijuana"). Morover, not all drivers consented to be tested, and the percentages are only for those who were tested.

Seems like a perfectly valid study, as far as it goes (though there's been some mild criticism of the test they used). It's mostly interesting as a study of how marijuana and cocaine use correlate with visible intoxication and sobriety test results. It's not a study of the prevalence of drugs on the road: the NHTSA report is right about that. The numbers it reports are useless in that context.

So the jump from that study to what and Roadshow implied -- that 45% of people involved in car accidents test positive for marijuana -- is quite a leap, and attributing that leap to the NHTSA seems especially odd since they explicitly say the study shouldn't be used for those purposes.

What really happened here?

So what happened here? Brookoff, Cook, Williams and Mann publish a study on behavior of reckless drivers under the influence of drugs.

NHTSA makes a brief and dismissive reference to it in a long survey paper.

Then writes an article that references the study but entirely misinterprets the numbers. This study gets picked up and referenced by other sites, out of context.

Then somehow the paragraph from shows up in Roadshow, attributed to the NHTSA. How did that happen?

If you look at the article, the paragraph cites Brookoff in its first sentence, then goes on to other unrelated claims, citing an NHTSA study at the end of the paragraph. I suppose it's possible (though hard to understand) that one could miss the first reference, and take the NHTSA reference at the end of the paragraph as the reference for the whole paragraph. That's the best guess I can come up with. Just another example of the game of telephone.

Nobody with any sense thinks it's a good idea to drive under the influence of marijuana or other intoxicants. But bogus statistics don't help make your point. They just cast doubt on everything else you say.

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[ 12:33 Jul 31, 2010    More headlines | permalink to this entry ]

Thu, 05 Jun 2008

Quote of the Week

From a BBC story on the wife of France's president:
She said her husband was so bright he appeared to have "five or even six brains".

Raises all kinds of intriguing followup questions, doesn't it?

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[ 20:46 Jun 05, 2008    More headlines | permalink to this entry ]

Wed, 20 Feb 2008

The mysterious vanishing planes

BBC was full of interesting news today.

Definitely the most interesting story was the one about the F-15 pilots rescued off Florida. It begins:

Two US fighter pilots have been rescued after their jets went missing over the Gulf of Mexico, the Air Force says.

Air Force spokeswoman Shirley Pigott said the pilots were rescued after their F-15C Eagles disappeared on a training mission.

The disappearance had triggered a search involving Coast Guard personnel, helicopters, planes and boats.

The Air Force has not yet determined if the planes collided or otherwise malfunctioned. The weather was clear.

Wow, that's quite a story! Not only do we have fighter planes disappearing in midair, but even after the pilots have been rescued, no one has any idea whether they collided.

[ 18:15 Feb 20, 2008    More headlines | permalink to this entry ]

Fri, 28 Oct 2005

A Value of "Close" With Which I was Previously Unfamiliar

A very strange article in today's SF Chronicle describes "Mysterious, bright lights in the night sky Wednesday that alarmed or bemused scores of Bay area residents".

It atributes to Andrew Fraknoi, chairman of the Foothill College astronomy department and media hound (it doesn't say that second part), information that "the lights were probably Mars and Venus, two planets that currently appear close together and will probably remain brilliant for another week or two until their orbits begin moving them away from the Earth again."

Aside from the "probably" (I was under the impression that the basic orbits of the major planets were fairly well understood, and that it's fairly rare that a planet suddenly deviates from its regular orbit in a visible way), I found this curious because Venus is currently in the early evening sky -- since its orbit lies inside that of the Earth, it can never appear to move very far from the Sun -- while Mars, a week before opposition, is rising in the early evening and overhead at roughly midnight.

Just to be sure, I checked with XEphem. The angular distance between Mars and Venus is current 146°. They're almost at opposite ends of the sky. This is a definition of "close" with which I was previously unfamiliar.

I don't know if Fraknoi really said this, or if he was simply misquoted by the reporter, David Perlman, the Chronicle's Science Editor. If so, the misquote is quite pervasive -- he repeats several times throughout the article Fraknoi's assurance that the lights (shown in a photograph accompanying the article, indeed close together though we aren't told anything about the lens used to take the photo) must be Venus and Mars.

Other giggle-inducing quotes from the article:

No one except astronomers could offer an explanation.
Well, gosh, you certainly wouldn't want to listen to those egghead astronomers about a question involving lights in the sky.

(Well, okay, in this case you shouldn't listen to them, because the Mars/Venus explanation obviously doesn't fit the observations.)

According to Fraknoi, Mars now far outshines even the brightest of all the stars in the sky, and when skies are clear, the fourth planet from the sun could look even bigger than normal.
Mars at opposition is certainly brighter than any star (except the Sun, of course). It currently shines with a magnitude of about -2.2 (a smaller number means a brighter object; the brightest star, Sirius, is magnitude -1.4. Venus, at the moment, is much brighter than either one at -4.2, as is usual since it's larger, closer, and more reflective than Mars. That might have been worth mentioning.

I can't figure out whether "even bigger than normal" is supposed to refer to size or brightness. Mars is normally a tiny object as viewed from Earth, too small to see much detail except for a few months around opposition every couple of years. Indeed it is much bigger than normal right now (and a lovely sight in a telescope!), as well as brighter; but "even bigger" seems like an odd phrasing for something normally so small.

But since Mars' size isn't visible except in a telescope, Dave thinks "bigger" here was meant to refer to brightness: the misconception that brighter objects look bigger. I shouldn't make fun of this: the great astronomer Tycho Brahe, in the 1500's, was convinced that stars had angular size instead of being point sources. He thought brighter stars appeared bigger, and based his geocentric solar system model on that. That view wasn't disproved until Galileo invented the telescope. It's a common misconception even today, but I'd hate to think the Chronicle's science editor was encouraging it, so I'll stick to the assumption that he really meant size and that the "even" was just an odd journalistic embellishment.

So what were the mysterious lights? I don't know. I didn't see them, and the article doesn't give enough detail to make a good guess. But the photo looks a lot like airplanes or helicopters; at least one of the lights has a couple of smaller lights to either side, usually a dead giveaway for an aircraft.

Update the following day: I wasn't the only one to complain about this article, and the Chron published a paragraph in the Corrections section this morning clarifying that Venus is nowhere near Mars and could not have been related to the lights people reported.

[ 10:57 Oct 28, 2005    More headlines | permalink to this entry ]

Fri, 14 Oct 2005

More On Low Orbits

Wacky Chinese orbital physics are in the BBC again. Today's story tells us how they've corrected yesterday's orbital problems. Quoting from China's "official Xinhua news agency", the BBC tells us:
Xinhua said the craft had deviated from its planned trajectory because of the Earth's gravitational pull.

I can hear them now ... "Darn it! I guess we forgot to take the earth's gravity into account when making our orbital calculations!"

[ 20:40 Oct 14, 2005    More headlines | permalink to this entry ]

Thu, 13 Oct 2005

Darn Those Low Orbits

BBC News Science tells us about the orbital problems of China's manned Shenzhou VI spacecraft.

Gravity has drawn Shenzhou VI too close to earth, the agency said.

Shenzhou VI, which has two astronauts on board, is in a low enough orbit to be affected by the Earth's gravitational pull.

Don't you hate those low orbits that are affected by gravity? Maybe next time they should choose an orbit high enough that it isn't subject to gravity.

[ 19:39 Oct 13, 2005    More headlines | permalink to this entry ]

Mon, 07 Mar 2005

Mukhtar Mai is a Hero

The acquittals in the Pakistan gang rape case are an outrage. You may have read about the case: a village tribunal in a remote area of Pakistan passed sentence that Mukhtar Mai be gang raped to punish her brother for an offense he allegedly committed (though most news reports indicated that he was not guilty of the offense, which was actually committed by one of the rapists. Not that that has any bearing on whether a wholly innocent woman should be raped for someone else's supposed crimes.)

The case spawned international outrage in a world previously unaware of the brutality of Pakistan's archaic tribunal system. The rapists were convicted and sentenced to death; but last week, their conviction was overturned.

Mukhtar Mai is a hero for standing up to them and continuing to press her case. I can't imagine what it must be like to be in her position. I am in awe of her. Mai's courage will help every woman in Pakistan, and in other countries with similar disregard for women's humanity. And not only that: she's using any financial gains from the case to build schools in her village. She's built two already.

Several of the BBC followup stories have mentioned that most women "sentenced" under this barbaric system, to be raped or otherwise mistreated for the supposed offenses of male members of their clan, accept their fate, "believing that tribal or feudal leaders are too powerful to resist and that the police and judicial systems are stacked against them." If anyone wonders why they might think that, last week's acquittal should answer any such questions rather handily.

None of the stories I've read anywhere goes into detail on the reason for the conviction having been overturned, besides the vague "lack of evidence". This seems odd considering all the reports of the original trial cited eyewitnesses. It's not clear why so few details are being reported. No one mentions the double standard which seems to be in place in Pakistan: where was the opportunity for Mai or her brother to appeal her outrageous punishment for his supposed crime?

The case will be appealed to a higher court, following international outrage at the current verdict. It is not yet clear whether the rapists will remain in prison until then.

[ 20:37 Mar 07, 2005    More headlines | permalink to this entry ]

Thu, 03 Mar 2005

Boondocks Pulled for Criticising Bush

Slate and Editor and Publisher report that several major newspapers have dropped Monday's Boondocks comic strip.

In the strip, one character reads from a newspaper, "Bush got recorded admitting that he smoked weed." Another character quips, "Maybe he smoked it to take the edge off the coke."

The best part of the story: the Chicago Tribune's given reason for censoring the comic was that it "presents inaccurate information as fact."

It's not clear which part of the comic was the inaccurate information presented as fact. The news about the tape recording in question, which was widely printed and has not been disputed by the White House? Or the quip in response, the one that starts with "maybe"?

If the Chicago Tribune is so worried about inaccurate information presented as fact ... does that mean that they will no longer be reporting on Bush's speeches and press releases?

[ 08:54 Mar 03, 2005    More headlines | permalink to this entry ]