Reply #5: good debunking of the methodology of this "study" here...

[paulsby]

http://volokh.com/2009/10/05/guns-did-not-protect-those-who-possessed-them-from-being-shot-in-an-assault/#comments


Conspicuously missing from the press release and the news story were two critical limitations that were admitted in the original study. These qualifiers mean that the press release headline, as well as all the other statements and implications of causation, were quite mistaken. Perhaps defensive possession and carrying of guns helps protect the possessor and carrier, and perhaps it doesn’t. But the study sheds virtually no light on the subject.

1. To begin with, there’s the obvious causation/correlation problem. Maybe, as the authors speculate, carrying a gun increases your chances of being shot with a gun (as suggested by the framing of the issue as “whether guns are protective or perilous”), or at least fails to decrease them (”guns did not protect”). Or maybe a third source — perhaps some people’s being the targets of death threats, or being in a dangerous legal line of work, or being gang members or drug dealers — causes both higher gun carrying among those people and higher risk of being shot.

By way of analogy, we don’t suggest that pacemakers cause heart attacks, or don’t protect against heart attacks, just because we find a correlation between the presence of pacemaker and the incidence of heart attacks. Obviously, people might get pacemakers precisely because they’re at risk of heart attacks. Well, people might get guns precisely because they’re at risk of attack. (Stewart Baker makes a similar point.) One can try to control for this in some measure — but while the study controls for some relevant attributes (race, sex, age, neighborhood, having a “high-risk occupation,” and having at least one arrest on one’s record), it leaves a vast range of factors uncontrolled. You’d think that gang members are more likely than others to carry guns and to get shot, even controlling for the presence of an arrest record. (Lots of law-abiding people carry guns, but I expect that more gang members do.) But the study doesn’t control for that, or for many other things.

Let me illustrate this with a deliberately oversimplified model. Let’s begin by assuming a total population of 100,000, that’s divided into two groups, a 10% high-risk group and a 90% low-risk group. Let’s say that the high-risk group has a 60% risk of being attacked, and as a result 40% of its members carry guns. And let’s say that the low-risk group has a 5% risk of being attacked, and as a result 3% of its members carry guns. Let’s also imagine a total population of 100,000 (just to make the numbers easier), and let’s assume that possessing a gun has a modest protective effect for both groups — it reduces the risk of being injured when attacked from 75% to 60%.

Here’s what this turns out yielding, with “A” meaning “armed subgroup” and “U” meaning the unarmed subgroup.

(graphic table follows...)

The result: The armed subgroup has 3.5 the risk of injury compared to the unarmed subgroup, and the relative odds ratio between them is 4.29. And this is so even though in the model gun possession decreases the injury risk for both the high- and the low-risk group.

Naturally, this is just a model; the real numbers are likely very different from the ones I give here, and in fact no-one knows what the real numbers are. (The model also doesn’t precisely fit the numbers in the study, though I’m pretty sure one can make a similar model that would fit them more closely.) My point is that one just can’t infer from an odds ratio of over 4 to the judgment that “guns did not protect those who possessed them,” much less that they were actually “perilous” to the possessors. The high odds ratio is just as consistent with the model I describe as with a model where gun possession increases the risk of injury...

more follows...