Following up on a report I blogged about a couple of days back, Jacob Sullum uses the same methodology to prove that skydivers would be better off without parachutes:

In Philadelphia, researchers at the University of Pennsylvania find, possessing a gun is strongly associated with getting shot. Since “guns did not protect those who possessed them,” they conclude, “people should rethink their possession of guns.” This is like noting that possessing a parachute is strongly associated with being injured while jumping from a plane, then concluding that skydivers would be better off unencumbered by safety equipment designed to slow their descent. “Can this study possibly be as stupid as it sounds?” asks Stewart Baker at Skating on Stilts. Having shelled out $30 for the privilege of reading the entire article, which appears in the November American Journal of Public Health, I can confirm that the answer is yes.

[. . .]

While the reseachers took into account a few confounding variables related to this tendency (including having an arrest record, living in a rough neighborhood, and having a high-risk occupation), they cannot possibly have considered all the factors that might make people more prone to violent attack and therefore more likely to have a gun as a defense against that hazard. To take just one example, not every criminal has an arrest record. Yet it seems fair to assume that criminals in Philadelphia are a) more likely than noncriminals to be armed and b) more likely than noncriminals to be shot. That does not mean having a gun increases their chance of being shot. Certainly they believe (as police officers do) that having a gun makes them safer than they otherwise would be. Nothing in this study contradicts that belief.

Of course, most people will only see the headline, so the underlying purpose of publishing the “study” has been achieved.

In no way should you try to read the data from this study as being anything other than unbiased and objective:

Medical researchers in Philadelphia have conducted out a study which indicates — according to their interpretation — that carrying a gun causes people to get shot more often. “People should rethink their possession of guns,” say the medics.

“This study helps resolve the long-standing debate about whether guns are protective or perilous,” says University of Pennsylvania epidemiology prof Charles Branas. The Penn announcement is headlined “Gun Possession [is] of questionable value in an Assault”, so it’s pretty clear which way he’s leaning.

The Penn researchers carried out their study by randomly selecting 677 people in Philadelphia who had been shot in “assaults”. Apparently five people sustain gunshot wounds every day in the City of Brotherly Love, so there were plenty to choose from.

According to the profs, six per cent of the shooting victims were packing heat when they got plugged. They compared that to a control sample of Philadelphians who had not been shot, and concluded that “people with a gun were 4.5 times more likely to be shot in an assault than those not possessing a gun”.

Of course, there’s no problem with basing your statistically valid sample on people who have already been shot: given the chance of being shot in Philadelphia, they could just have gone round to a few local bars and found the same numbers, right?

You know that the study has a certain, um, preference, when even the folks at The Register are pointing out that the data may not be randomly selected:

There didn’t seem to be any account taken of the fact that people with good reason to fear being shot — for instance drug dealers, secret agents etc — would be more likely to tool up than those with no such concerns.

The profs’ reasoning, however, would seem to be that if someone sticks you up in the street and you haven’t got a gun, you’ll just hand over your valuables and so escape with a whole skin. If you’ve got a gat, however, you might try to draw it and so get shot. Tactically, of course, it might be wiser to first hand over your wallet and then craftily backshoot the robber as he departed, but no matter.