BMC Public Health 2009, 9:199 http://www.biomedcentral.com/1471-2458/9/199 1,000 in 1993 to 0.70 per 1,000 in 1999. The average amount by which gun homicide rates and FFL prevalence rates decreased over the seven-year period was approxi- mately equal (23% and 22%, respectively). Table 1 shows characteristics of U.S. counties. The rate of gun homicide ranged from 0.00 per 100,000 to 187.27 per 100,000 and the rate of nongun homicide ranged from 0.00 per 100,000 to 97.75 per 100,000, with average gun and nongun homicide rates of 3.25 per 100,000 and 1.38 per 100,000, respectively. The number of FFLs ranged from 0.001 per 1,000 to 30.93 per 1,000 with an average of 1.15 per 1,000. There were no counties without FFLs; each county had at least one FFL active during each study year. Table 2 shows results of the overall statistical model, which adjusted for covariates using regression but did not permit for the possibility that the relation between the prevalence of FFLs and gun homicide rates could vary over the years of the study period and according to county type (i.e., urbanization). The variable for percent of female headed households was excluded during the model build- ing process due to multicolinearity. The results based on this overall model suggested that the prevalence of FFLs in U.S. counties was not associated with the rate of gun hom- icide in the county (RR = 0.98, 95% confidence interval [CI] 0.93 to 1.04) (Table 2). However, inclusion of an interaction term revealed that the relation between FFLs and gun homicide was found to vary significantly by urbanization (p < 0.01). The subsequent modeling revealed that in major cities, a disproportionately high prevalence of FFLs was associated with significantly higher gun homicide rates (RR = 1.70, 95% CI 1.03 to 2.81) (Table 3). Additionally, the magnitude of this association increased significantly over the study period with an aver- age increase of 90% per year (p < 0.001) (Figure 2). By contrast, in other cities and in suburbs a disproportion- ately high prevalence of FFLs was associated with signifi- cantly lower gun homicide rates (RR = 0.73, 95% CI 0.57 to 0.93; RR = 0.87, 95% CI 0.77 to 0.97, respectively), and the magnitude of these associations did not change when tested for trend over the study period. The prevalence of FFLs was not associated with gun homicide rates in small towns. Inclusion of the covariate representing county- level rates of nongun homicide did not substantively change the results. Also, the results did not change sub- stantively when the variable that was used to adjust for the impact of the prevalence of FFLs in surrounding counties was excluded from the models. The results of the models run with FS/S used in place of the FFL prevalence variable (not presented) were generally consistent with our findings, although the link between gun availability and gun homicide appeared to be even stronger (positive) and more widespread than seen in the Table 2: Gun homicide rates as a function of the number of federal firearms licensees in U.S. counties FFLs FFLs in surrounding counties Year % poverty % married % college educated % African American % Native American % Hispanic % 15–24 years old % male % living alone % of arrests that were drug related Drug arrest jurisdiction adjustment Hospital beds Average per capita income Unemployment rate Leniency of state gun laws Major cities Other cities Suburbs Small towns (reference) Results of generalized linear negative binomial regression. SE denotes standard error. CI denotes confidence interval. Incident rate ratio 0.98 1.03 0.96 1.05 1.04 1.00 1.03 1.01 1.01 0.95 1.02 1.02 1.02 0.92 1.00 1.00 1.01 1.01 1.97 1.39 1.27 -- SE 0.027 0.009 0.009 0.004 0.005 0.003 0.001 0.003 0.002 0.432 0.009 0.006 0.003 0.037 0.000 0.000 0.006 0.001 0.226 0.117 0.045 P-value 0.482 0.001 0.000 0.000 0.000 0.699 0.000 0.006 0.000 0.913 0.075 0.000 0.000 0.030 0.000 0.096 0.331 0.000 0.000 0.000 0.000 95% CI 0.93, 1.04 1.01, 1.05 0.95, 0.98 1.05, 1.06 1.03, 1.05 0.99, 1.01 1.03, 1.03 1.00, 1.01 1.00, 1.01 0.39, 2.32 1.00, 1.03 1.01, 1.04 1.01, 1.02 0.85, 0.99 1.00, 1.00 1.00, 1.00 0.99, 1.02 1.01, 1.01 1.57, 2.46 1.17, 1.63 1.19, 1.37 Page 5 of 10 (page number not for citation purposes)