"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."
How does this only look at people carrying a gun?
"Their study assessed risk for being assaulted and then shot, a compound outcome event whose second element (being shot) is not inevitable given the first (being assaulted). Persons who were assaulted but not shot are not studied. We do not know whether any association between firearm possession and their outcome measure applies to assault, to being shot given an assault, or both."[1]
[1]http://www.ncbi.nlm.nih.gov/pmc/articles/PMC2866589/ [2]http://reason.com/blog/2009/10/05/why-skydivers-would-be-bet... [3]http://volokh.com/2009/10/05/guns-did-not-protect-those-who-...