So Charles Branas's team at the University of Pennsylvania analysed 677 shootings over two-and-a-half years to discover whether victims were carrying at the time, and compared them to other Philly residents of similar age, sex and ethnicity.
This study only looked at one side of the equation of people who carry guns, the ones who got shot. What about all the people who carry guns and never use them? Wouldn't they be excluded from this study?
"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-...