As I can tell the study
did not differentiate lawful carry from criminal carry. That's pretty telling, isn't it?
A summary at
http://phys.org/news173531867.html#jCp from 2009 only said this about it:
Penn researchers investigated the link between being shot in an assault and a person's 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.
I wonder how many of the people in this study who were "possessing a gun at the time of the shooting" were either gangbangers who were shot by other gangbangers, or armed robbers, home invaders or other types of criminals who were shot by cops or lawfully armed citizens, or even cops shot by criminals, for that matter.
Unfortunately, I've searched, but I can't find the study, to err, study... However, an epidemiologist with a bias could take any study and make it look bad. If they're a tenured PhD they won't even get a raised eyebrow if they took a sample of two people and said that anyone who drank Philly's tap water was 100% more likely to contract brain cancer, because the other one only drank soda all day long, and died during a diabetic coma before the study concluded.