Volokh: Criticism of "Guns Did Not Protect Those Who Possessed Them from Being Shot in an Assault
UCLA Law Professor Eugene Volokh, writing on his blog, The Volokh Conspiracy, recently critiqued the bad science and mistaken statistical conclusions in a press release posted on ScienceDaily.com. The release, entitled "Guns Did Not Protect Those Who Possessed Them from Being Shot in an Assault, is the title of a misleading study by Charles C. Branas, PhD, Associate Professor of Epidemiology at Penn.
The Branas study estimated that people with a gun were 4.5 times more likely to be shot in an assault than those not possessing a gun.
This study helps resolve the long-standing debate about whether guns are protective or perilous, notes study author Charles C. Branas, PhD, Associate Professor of Epidemiology. Will possessing a firearm always safeguard against harm or will it promote a false sense of security?
Penn researchers investigated the link between being shot in an assault and a persons possession of a gun at the time of the shooting. As identified by police and medical examiners, they randomly selected 677 cases of Philadelphia residents who were shot in an assault from 2003 to 2006. Six percent of these cases were in possession of a gun (such as in a holster, pocket, waistband, or vehicle) when they were shot.
These shooting cases were matched to Philadelphia residents who acted as the studys controls. To identify the controls, trained phone canvassers called random Philadelphians soon after a reported shooting and asked about their possession of a gun at the time of the shooting. These random Philadelphians had not been shot and had nothing to do with the shooting. This is the same approach that epidemiologists have historically used to establish links between such things as smoking and lung cancer or drinking and car crashes.
This was promptly echoed in the Philadelphia Daily News.
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 doesnt. But the study sheds virtually no light on the subject.
1. To begin with, theres 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 peoples 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 dont suggest that pacemakers cause heart attacks, or dont 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 theyre at risk of heart attacks. Well, people might get guns precisely because theyre 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 ones record), it leaves a vast range of factors uncontrolled. Youd 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 doesnt control for that, or for many other things.
Let me illustrate this with a deliberately oversimplified model.
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