BMC Public Health 2009, 9:199 http://www.biomedcentral.com/1471-2458/9/199 Table 3: Gun homicide rates (per 100,000 population) as a function of the prevalence of federal firearms licensees (FFLs) (per 1,000 population) in U.S. counties, by county type and year, 1993–1999 1993–1999 1993 1993 1995 1996 1997 1998 1999 Incident rate ratio (95% CI) All counties 0.98 (0.93, 1.04) 1.09 (1.02, 1.16) 1.03 (0.91, 1.15) 0.84 (0.73, 0.94) 0.95 (0.80, 1.13) 0.96 (0.80, 1.15) 0.92 (0.73, 1.15) 0.85 (0.67, 1.07) Major cities 1.70 (1.03, 2.81) 1.69 (0.98, 2.90) 1.65 (0.72, 3.77) 2.16 (0.65, 7.20) 2.58 (0.33, 19.99) 3.12 (0.22, 44.78) 11.24 (0.46, 277.38) 12.72 (0.64, 253.19) Incident rate ratio (95% CI) Other cities 0.73 (0.57, 0.93) 0.86 (0.67, 1.11) 0.62 (0.42, 0.92) 0.30 (0.17, 0.53) 0.64 (0.08, 5.10) 0.28 (0.08,0.96) 0.29 (0.10, 0.87) 0.51 (0.13, 2.00) Suburbs 0.87 (0.77, 0.97) 0.99 (0.84, 1.18) 0.78 (0.63, 0.96) 0.65 (0.52, 0.82) 0.71 (0.47, 1.08) 0.48 (0.30, 0.76) 0.48 (0.31, 0.72) 0.62 (0.38, 1.01) Small towns 1.00 (0.95, 1.05) 1.11 (1.04, 1.18) 1.07 (0.96, 1.19) 0.87 (0.77, 0.98) 0.99 (0.83, 1.17) 1.03 (0.87, 1.13) 0.98 (0.79, 1.22) 0.89 (0.70, 1.12) Results of generalized linear negative binomial regression models adjusted for covariates. CI indicates confidence interval. analysis presented here. Importantly though, FS/S was strongly (positively) correlated with FFL prevalence in major cities (Spearman correlation coefficient = 0.67) but was weakly correlated and not correlated with FS/S in other cities (0.32), suburbs (0.16), or small towns (0.07), respectively. From this evidence it appears that FFL preva- lence is a good proxy for firearm availability in major cit- ies only, suggesting that the findings for how FFL prevalence relates to gun homicide may be valid for the findings pertaining to major cities but not to counties of other types. Discussion Our analyses provide evidence of an association between the per capita rate of licensed firearm dealers in a county and its rate of firearm homicide. In particular, we found that having a disproportionately high number FFLs was associated with significantly higher rates of firearm homi- cide in major cities. As such, FFLs may represent a risk fac- tor for gun homicide that is modifiable. To the best of our knowledge, the association between licensed gun dealers and homicide rates has not previously been estimated. We also found more FFLs to be associated with significantly lower firearm homicide rates in other cities and in sub- urbs. Possible explanations for both findings are dis- cussed below. Our evaluation strategy followed the assumption that the number of FFLs in a county gives some indication of the prevalence of firearms (i.e., a proxy of gun availability). Although no available data report the number of guns sold per dealer, the basic finding that major cities having the most FFLs per capita also have the highest rates of gun homicide is consistent with what is known about how FFLs and communities relate in terms of gun availability. Data from gun traces (a determination of the chain of ownership, usually conducted in connection with a crim- inal investigation) conducted by the ATF may provide the best insight. Between July 1996 and December 1998, the ATF conducted 1,530 trace investigations to determine whether guns used during crimes were trafficked from an FFL into the illegal gun market and to determine the point of first purchase.[8] FFLs accounted for less than 10 per- cent of the 1,530 trace investigations but for nearly half (40,000) of all firearms involved in these traces. The aver- age number of firearms trafficked by the FFLs under inves- tigation was 350, which far exceeds the average number of firearms trafficked by other means including gun shows (130 guns), unlicensed gun dealers (75 guns), and straw purchasers (37 guns). The large volume of firearms that can be obtained by FFLs is possibly what underlies this discrepancy and why FFLs may figure prominently as a risk factor. In addition, a study where researchers tele- phoned FFLs and posed as customers provides additional evidence of how FFLs can facilitate the flow of guns to criminals. Gun dealers were generally willing to sell a handgun even when the buyer indicated an intention to purchase the gun illegally on behalf of someone else.[30] Other ATF data provide additional support for the possi- bility that gun homicide is a function of local FFLs. Guns are often found to have been used for criminal purposes Page 6 of 10 (page number not for citation purposes)