Epidemiology of Gun Violence in the City of Brotherly Love

A recent paper from the American Journal of Public Health takes a look at gun possession and gun assault in Philadelphia. From the abstract:

Objectives. We investigated the possible relationship between being shot in an assault and possession of a gun at the time.

Methods. We enrolled 677 case participants that had been shot in an assault and 684 population-based control participants within Philadelphia, PA, from 2003 to 2006. We adjusted odds ratios for confounding variables.

Results. After adjustment, individuals in possession of a gun were 4.46 (P<.05) times more likely to be shot in an assault than those not in possession. Among gun assaults where the victim had at least some chance to resist, this adjusted odds ratio increased to 5.45 (P<.05).

The paper also had some stats on gun violence that were surprising, though I admit I don't know much about gun statistics. During the study period (2003-2006) there were 3485 shootings in Philly (Population 1.4M, metro 5.8M) or an average of 4.77 shootings per day and an average of 9 shooting free days per year. I wonder how that compares to a city like Toronto (Population 2.5M, metro 5.5M).

The study was done by taking a case-control approach. Case participants were enrolled via the police department and excluded accidental, unintentional, self-inflicted and police-related shootings - the study interest was assault with a firearm. Control participants were matched according to age, race (black or white only), gender, and time of shooting. In other words, the researchers took people who were shot, matched with people who were not then looked at whether each was in possession of a gun or not. After their analysis they conclude:

On average, guns did not protect those who possessed them from being shot in an assault. Although successful defensive gun uses occur each year, the probability of success may be low for civilian gun users in urban areas. Such users should reconsider their possession of guns or, at least, understand that regular possession necessitates careful safety countermeasures.

However, there were some flaws in the study. For example, the authors "assumed that the resident population of Philadelphia risked being shot in an assault at any location and at any time of day or night", nor do they look at the legality of the guns in question. As such, matching wasn't based on location and thus excludes, for example, the possibility of 'bad neighbourhoods', it also excludes the possibility that people with illegal firearms may be involved in other illegal activities that increase the risk of a shooting. This despite their finding that people being shot were more likely to be in areas with lower income and more illicit drug trafficking.

This, of course, doesn't invalidate any results, but it does make it harder to suggest causation as strongly as the authors do in their discussion. From these results it's difficult to say that carrying a firearm increases your risk of assault vs. being in a high-risk group (eg. living in a certain area) increasing the chances of carrying a gun.

Still, the fact remains that gun possession is an indicator of assault risk (even if not necessarily the cause) and offers no guarantee of protection from being shot in an assault.