BMC Public Health 2009, 9:199 http://www.biomedcentral.com/1471-2458/9/199 Second, to the best of our knowledge, our study, in focus- ing on gun dealers as a potential risk factor for homicide, is the first to assess a tangible measure of gun availability that can be modified as part of prevention activities. Law enforcement, city planners, and legal strategists in cities with high gun homicide rates can concretely focus in on excessive or problem gun dealers as opposed to the more nebulous issue of "gun availability." Moreover, local efforts to close down illegal gun commerce have already shown the potential to be effective.[33,40,41] As one example, zoning laws, which control the location and operation of stores and individual dealers licensed to sell firearms, have been used to regulate locations of FFLs in several U.S. communities.[11,12,42] An attempt to launch a coordinated effort to identify and act on prob- lematic gun dealers will surely face challenges, however. For one, a key component of such efforts will be the polic- ing activities of the ATF which, in having inspected fewer than 10% of FFLs in each year since 1979 and fewer than 5% of FFLs in most of those years, may have resources insufficient for the task.[6] Our analysis also had limitations. As discussed, we could not account for the actual volume of firearms introduced by each FFL into the community. Although gun sales data are not currently available for a more focused test, the recent National Academy of Sciences report called for bet- ter information on FFLs to be collected and made availa- ble for research.[39] Additionally, we did not explicitly accommodate spatial autocorrelation in the estimation of the FFL effect estimates. Refitting the models presented in Table 2 and Table 3 including a simultaneous autoregres- sive (SAR) structure,[43] assuming that correlation declined in proportion to the square of the distance between counties, generally changed coefficients by less than 5%. Also, data for FFLs for years after 1999 were not available. Thus we could not analyze a more recent period. Nevertheless, we do not anticipate that the rela- tion observed here between FFLs and homicide would have changed since the study period and thus this charac- teristic of the data should not be interpreted as devaluing the findings. Finally, it may be that high rates of homicide may lead to increased demand for firearms and hence additional FFLs, in which case the results of the "FFL as risk factor" hypothesis that our models have been designed to test would be spurious. A stronger analytic approach would be to test whether within-county increases in FFL prevalence were followed by increases in the rate of gun homicide. We considered that approach, but found many instances in which a county experienced no gun homicides in certain years but some homicides in the subsequent year, which prevents an annual change in homicide rate from being calculated. Also, as discussed above, we found evidence that FFL prevalence became a better proxy for firearm availability over time, which led to our preference for the 1999 models and our judgment to refrain from including change models in the current manuscript. An instrumental variable approach could be pursued as well, to attempt to remove from the models the influence of circularity that may exist. We hope this manuscript will inform how such design alternatives may be approached, and acknowledge their need given the cross-sectional nature of the present study. Conclusion If locations of retailers licensed to sell firearms are indeed functioning as a spigot through which deadly firearms flow into criminal hands, then communities with greater geographic access to these dealers should ostensibly expe- rience more firearm homicides. Our findings are consist- ent with the hypothesis that this is occurring in major U.S. cities. The modification of FFLs, as tangible entities that are tracked and overseen at the national level and, in some cases, at the state and local levels, may be a feasible inter- vention to reduce firearm homicide. Competing interests The authors declare that they have no competing interests. Authors' contributions CB, MN and ME conceived of the study and supervised all aspects of its implementation. ME and RK computed var- iables involved in the spatial component of the analysis and DW and ME conducted the statistical analysis. DW and CB drafted the manuscript. All authors helped to con- ceptualize ideas, interpret findings, and revise drafts of the manuscript. Acknowledgements Funded through grants from the National Institute on Alcohol Abuse and Alcoholism (R01 AA013119), the National Institute of Justice (Project Safe Neighborhoods), and the Joyce Foundation. References 1. CDC: WISQARS Leading cause of death reports, 2005. 2008 [http://www.cdc.gov/injury/wisqars/]. 2. Fox JA, Zawitz MW: Weapons used (1980–2005). In: Homicide trends in the U.S. 2008 [http://www.ojp.usdoj.gov/bjs/homicide/ tables/weaponstab.htm]. 3. Diaz T: Making a killing: the business of guns in America New York, NY: New York Press; 1999. 4. Koper CS: Federal legislation and gun markets: how much have recent reforms of the federal firearms licensing system reduced criminal gun suppliers? Criminology and Public Policy 2002, 1(2):151-178. 5. Wintemute GJ: Relationship between illegal use of handguns and handgun sales volume. JAMA 2000, 284(5):566-567. 6. ATF: Firearms commerce in the United States. Washington, DC: U.S. Department of Justice, Bureau of Alcohol, Tobacco and Fire- arms; 2002. 7. Cook PJ, Ludwig J: Guns in America: national survey on private ownership and use of firearms (NCJ 165476) Washington, DC: U.S. Department of Justice, National Institute of Justice; 1997. 8. ATF: Following the gun: enforcing federal laws against firearms traffickers Washington DC: Department of the Treasury, Bureau of Alcohol, Tobacco & Firearms; 2000. Page 9 of 10 (page number not for citation purposes)