In-State and Interstate Associations Between Gun Shows and Firearm Deaths and Injuries: A Quasi-experimental Study

Ellicott C. Matthay; Jessica Galin; Kara E. Rudolph; Kriszta Farkas; Garen J. Wintemute; Jennifer Ahern

doi:10.7326/M17-1792

Original Research |19 December 2017

In-State and Interstate Associations Between Gun Shows and Firearm Deaths and Injuries: A Quasi-experimental Study Free

Ellicott C. Matthay, MPH; Jessica Galin, MPH; Kara E. Rudolph, PhD, MPH, MHS; Kriszta Farkas, MPH; Garen J. Wintemute, MD, MPH; Jennifer Ahern, PhD, MPH

Article, Author, and Disclosure Information

This article was published at Annals.org on 24 October 2017.

From University of California, Berkeley, School of Public Health, Berkeley, and University of California, Davis, Sacramento, California.
Disclaimer: The analyses, interpretations, and conclusions of this report are attributable to the authors and not to the California Department of Public Health or National Institutes of Health.
Acknowledgment: The authors thank the following funding sources: the Eunice Kennedy Shriver National Institute of Child Health and Human Development; the National Institutes of Health Office of the Director; the University of California, Berkeley Committee on Research; and the Heising-Simons Foundation.
Grant Support: By grant DP2HD080350 from the Eunice Kennedy Shriver National Institute of Child Health and Human Development and the National Institutes of Health Office of the Director (Ms. Matthay, Ms. Galin, Ms. Farkas, and Dr. Ahern); the University of California, Berkeley Committee on Research (Ms. Matthay, Ms. Galin, Ms. Farkas, and Dr. Ahern); and grant 2016-219 from the Heising-Simons Foundation (Dr. Wintemute).
Disclosures: Ms. Matthay, Ms. Galin, Ms. Farkas, and Dr. Ahern report grants from the National Institutes of Health and University of California, Berkeley, during the conduct of the study. Dr. Wintemute reports grants from Heising-Simons Foundation during the conduct of the study. Disclosures can also be viewed at www.acponline.org/authors/icmje/ConflictOfInterestForms.do?msNum=M17-1792.
Editors' Disclosures: Christine Laine, MD, MPH, Editor in Chief, reports that she has no financial relationships or interests to disclose. Darren B. Taichman, MD, PhD, Executive Deputy Editor, reports that he has no financial relationships or interests to disclose. Cynthia D. Mulrow, MD, MSc, Senior Deputy Editor, reports that she has no relationships or interests to disclose. Deborah Cotton, MD, MPH, Deputy Editor, reports that she has no financial relationships or interest to disclose. Jaya K. Rao, MD, MHS, Deputy Editor, reports that she has stock holdings/options in Eli Lilly and Pfizer. Sankey V. Williams, MD, Deputy Editor, reports that he has no financial relationships or interests to disclose. Catharine B. Stack, PhD, MS, Deputy Editor for Statistics, reports that she has stock holdings in Pfizer and Johnson & Johnson.
Reproducible Research Statement:Study protocol: Not applicable. Statistical code: Available from Ellicott Matthay (e-mail, ematthay@berkeley.edu). Data set: Death and hospital visit data are available from the California Department of Public Health Vital Records and the Office of Statewide Health Planning and Development; data on dates and locations of gun shows are available from Ellicott Matthay (e-mail, ematthay@berkeley.edu).
Requests for Single Reprints: Ellicott C. Matthay, MPH, 50 University Hall, University of California, Berkeley, Berkeley, CA 94704; e-mail, ematthay@berkeley.edu.
Current Author Addresses: Ms. Matthay, Ms. Galin, Drs. Rudolph and Ahern, and Ms. Farkas: 50 University Hall, University of California, Berkeley, Berkeley, CA 94704.
Dr. Wintemute: Violence Prevention Research Program, UC Davis Medical Center, 2315 Stockton Boulevard, Sacramento, CA 95817.
Author Contributions: Conception and design: E.C. Matthay, J. Galin, K.E. Rudolph, G. Wintemute, J. Ahern.
Analysis and interpretation of the data: E.C. Matthay, J. Galin, K.E. Rudolph, G. Wintemute, J. Ahern.
Drafting of the article: E.C. Matthay, J. Galin, J. Ahern.
Critical revision of the article for important intellectual content: E.C. Matthay, K.E. Rudolph, K. Farkas, G. Wintemute, J. Ahern.
Final approval of the article: E.C. Matthay, J. Galin, K.E. Rudolph, K. Farkas, G. Wintemute, J. Ahern.
Provision of study materials or patients: G. Wintemute.
Statistical expertise: E.C. Matthay, K.E. Rudolph.
Obtaining of funding: G. Wintemute, J. Ahern.
Administrative, technical, or logistic support: E.C. Matthay, J. Galin.
Collection and assembly of data: E.C. Matthay, J. Galin, K. Farkas, G. Wintemute, J. Ahern.

Abstract

Background:

Gun shows are an important source of firearms, but no adequately powered studies have examined whether they are associated with increases in firearm injuries.

Objective:

To determine whether gun shows are associated with short-term increases in local firearm injuries and whether this association differs by the state in which the gun show is held.

Design:

Quasi-experimental.

Setting:

California.

Participants:

Persons in California within driving distance of gun shows.

Measurements:

Gun shows in California and Nevada between 2005 and 2013 (n = 915 shows) and rates of firearm-related deaths, emergency department visits, and inpatient hospitalizations in California.

Results:

Compared with the 2 weeks before, postshow firearm injury rates remained stable in regions near California gun shows but increased from 0.67 injuries (95% CI, 0.55 to 0.80 injuries) to 1.14 injuries (CI, 0.97 to 1.30 injuries) per 100 000 persons in regions near Nevada shows. After adjustment for seasonality and clustering, California shows were not associated with increases in local firearm injuries (rate ratio [RR], 0.99 [CI, 0.97 to 1.02]) but Nevada shows were associated with increased injuries in California (RR, 1.69 [CI, 1.16 to 2.45]). The pre–post difference was significantly higher for Nevada shows than California shows (ratio of RRs, 1.70 [CI, 1.17 to 2.47]). The Nevada association was driven by significant increases in firearm injuries from interpersonal violence (RR, 2.23 [CI, 1.01 to 4.89]) but corresponded to a small increase in absolute numbers. Nonfirearm injuries served as a negative control and were not associated with California or Nevada gun shows. Results were robust to sensitivity analyses.

Limitation:

Firearm injuries were examined only in California, and gun show occurrence was not randomized.

Conclusion:

Gun shows in Nevada, but not California, were associated with local, short-term increases in firearm injuries in California. Differing associations for California versus Nevada gun shows may be due to California's stricter firearm regulations.

Primary Funding Source:

National Institutes of Health; University of California, Berkeley; and Heising-Simons Foundation.

Firearms are a leading cause of morbidity and mortality in the United States and accounted for more than 36 000 deaths and nearly 85 000 injuries in 2015 (1). Ownership increases the risk for suicide, homicide, and unintentional firearm death and injury in the home (2–8). Greater availability and ownership of firearms also contributes to the higher rate of firearm deaths and injuries (hereafter called “firearm injuries”) in the United States than in other high-income countries (9–12). Gun shows account for 4% to 9% of annual firearm sales (13–15) and 3% of gun owners' most recent gun acquisitions (16). However, many of these transfers do not involve a background check (16) and firearms from gun shows are disproportionately implicated in crimes (17, 18). Little is known about how gun shows contribute to firearm injuries in the United States.

More than 4000 gun shows are held annually in the United States (19). These shows, which can attract thousands of attendees and hundreds of sellers, generate a temporary and diverse source of new and used firearms, ammunition, and related equipment in a competitive market where sales may be subject to less oversight (15, 20). Consequently, gun shows may increase local firearm ownership and use and affect subsequent rates of firearm injury. State regulations also differ markedly, which may modify the association between gun shows and firearm injuries. In particular, interstate activity and the flow of firearms from less to more restrictive states have been documented previously (21), and this pattern, which may limit the effectiveness of regulations in states that have them, may also extend to gun shows.

Using a quasi-experimental, difference-in-differences design, we exploited the natural variation in the timing and location of gun shows to investigate whether they are associated with increased injury rates and whether this association varies by the state in which the gun show is held. California has some of the most restrictive firearm laws in the country, including a comprehensive set of statutes regulating gun shows (22, 23). In contrast, Nevada has some of the least restrictive laws in the country and no explicit regulations on gun shows (22). Thus, comparing pre–post differences in California and cross-border differences between California and Nevada gun shows may provide useful information on these different policy environments.

No formal evaluations have assessed the effects of policies regulating gun shows. Observational evidence from 5 states suggests that such activities as anonymous and undocumented sales are less frequent in California, where they are prohibited, than in states where they are legal (24). Previous evidence has also linked firearms purchased at gun shows to crimes (17, 18), but to our knowledge, only 1 study has examined the association between gun shows and subsequent firearm injuries. Duggan and colleagues (25) examined weekly violent firearm deaths in ZIP codes in the immediate vicinity of gun shows in California and Texas. They found no association and suggested that California's gun show regulations have no effect on violent firearm deaths. However, the study was criticized as having low statistical power, incomplete identification of gun shows, and an analytic approach ignoring California's requirement that buyers wait 10 days between purchasing and obtaining a firearm (26, 27).

We addressed these gaps while assessing whether firearm injuries increase in nearby California areas immediately after gun shows in California and Nevada. We hypothesized that gun shows lead to increased rates of firearm injury.

Methods

Overall Approach

We used a quasi-experimental, difference-in-differences design (28, 29). First, we compared firearm injury rates for the 2 weeks immediately before and after each gun show in California regions within convenient traveling distance of the show. Then, we compared this difference for the California populations exposed to California versus Nevada gun shows. This approach is advantageous because each region's characteristics, other than the occurrence of a show, are unlikely to change appreciably over so short a time. Thus, each region serves as its own control, allowing us to adjust for community-level characteristics that may be associated with firearm injuries.

Firearm Injuries

We identified fatal and nonfatal firearm injuries in California between 2005 and 2013 using death records from the California Department of Public Health Vital Records and emergency department and inpatient hospitalization records from the Office of Statewide Health Planning and Development. External cause-of-injury coding in California's hospital discharge records is mandatory, subject to ongoing quality assurance measures, and considered 100% complete (30). Emergency department records from before 2005 are not available.

Gun Show Data

We compiled dates and locations of gun shows in California and Nevada between 2005 and 2013 using published lists in the Big Show Journal. This source was the most comprehensive; other magazines (Gun and Knife Show Calendar and Gun List Magazine) and online sources (we considered 11 major Web sites) did not cover the entire study period or included fewer listings (95% vs. 65% coverage). We used ABBYY FineReader 12 character recognition software to convert scanned images of show listings to electronic alphanumeric data (31).

Database Construction

Regions considered local to gun shows were determined using the Google Maps Distance Matrix API (32) by measuring the typical driving time between each ZIP code centroid in California and each geocoded gun show location. Little evidence exists on how far or how long the effects of gun shows extend (27). Thus, we selected reasonable time frames and travel distances to balance capturing short-term effects with estimating stable rates and to include regions likely to be affected by gun shows while excluding regions so distant that unrelated firearm injuries might obscure potential relationships. We tested the sensitivity of our results to chosen time frames and travel times in several sensitivity analyses. “Before” periods were the 14 days before each show; “after” periods were the 14 days after the 10-day waiting period from the start of the show for California or after the start of the show for Nevada, which has no waiting period. ZIP code centroids within a 60- or 120-minute drive were considered to be within traveling distance of California and Nevada shows, respectively. In California, most persons can access a gun show within a 60-minute drive every few weeks, but we hypothesized that some persons in California would be willing to travel farther to Nevada's comparatively unregulated environment.

ZIP codes were occasionally local to several gun shows at the same time. This was problematic when the “before” period of a later gun show (show B) overlapped with the “after” period of an earlier show (show A). Without consideration of this overlap, the ZIP code would be misclassified as “unexposed” for examination of show B when it was “exposed” to show A. In these cases, we excluded the overlapping ZIP code from analyses of show B (hereafter “overlap exclusions”). Restricting to ZIP codes far enough from the border to eliminate the need for overlap exclusions did not alter the findings (results available on request). Throughout, rates are reported per 100 000 persons in regions within traveling distance of shows.

Statistical Analysis

We did a difference-in-differences analysis (28, 29) at the gun show–period level using multivariable Poisson mixed-effects regression. The main outcome measure was the rate of firearm injury. The full model specification was as follows:

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where y_tks was the count of firearm injuries at time t in the region surrounding gun show s in state k; β₀, the intercept; X_t, the period (after vs. before); X_k, the state of the show (Nevada vs. California); X_m, month indicators to account for seasonality; ρ_cs and ρ_c, random-effects intercepts to account for clustering by gun shows nested within cities; log(d_tsk), an offset for the number of at-risk persons; and ϵ_tsk, the error term. Statistical testing of the dispersion parameter indicated that a Poisson model was more appropriate than a negative binomial model. Under this specification, exp(β₁) estimates the rate ratio (RR) associated with gun shows in California, exp(β₁ + β₃) estimates the RR associated with gun shows in Nevada, and exp(β₃) estimates the difference-in-differences estimate—the ratio of RRs—capturing the increase in firearm injury rates after Nevada shows compared with that after California shows.

P values less than 0.05 were considered statistically significant. Data were processed by using SAS, version 9.3 (SAS Institute), and R, version 3.2.1 (R Foundation), and regression analysis was done using the lme4 package (33) in R, version 3.2.1. This study was approved by the State of California and University of California, Berkeley, Committees for the Protection of Human Subjects.

Subgroup and Secondary Analyses

To examine variation by firearm injury type, we did subgroup analyses for intentional interpersonal violence, intentional self-harm, unintentional injuries, and injuries of undetermined intent (Appendix Table 1). Because the exposure periods and geographic regions defined for California and Nevada shows were not identical (with vs. without a waiting period; a 60- vs. a 120-minute drive), we also stratified the analysis by state. In addition, we did analyses restricted to specific gun shows and affected regions to examine potential associations along known firearm trafficking routes between Reno and San Francisco and between Las Vegas and Los Angeles. We also tested the association between California gun shows and California firearm injuries ignoring the 10-day waiting period, because activities other than legal firearm purchases (such as ammunition or parts purchases, illegal purchases, and repairs) may affect firearm injuries and do not have a waiting period. We tested the association between gun shows and nonfirearm injuries as a negative control to assess whether common causes of firearm and nonfirearm injuries confounded our findings (34).

Appendix Table 1. ICD-9 and ICD-10 External Cause-of-Injury Codes Used to Identify and Classify Firearm Deaths and Injuries

Finally, differences in associations between California and Nevada gun shows may be due to differing characteristics of regions exposed to California shows versus those exposed to Nevada shows. To address this potential source of variation, we restricted the entire analysis to regions similar to those exposed to Nevada gun shows. We tightly matched on ZIP code characteristics that differed between regions exposed to California versus Nevada shows and may modify the association between gun shows and firearm injuries (35, 36). These were population density, percentage of veterans, median income, median age, percentage of white non-Hispanic persons, hunting licenses per capita, and the overall rate of firearm injury between 2005 and 2013. We tested a range of matching approaches, all of which produced similar matches. Further details on the matching approach and characteristics of this restricted analysis are in the Appendix.

Power Calculations

To confirm that our study had sufficient statistical power, we did a power analysis using simulated data that were generated to be similar to the observed data (Appendix Figure 1) (37). We applied the main analysis regression approach to each simulated data set and recorded the proportion of simulations with a significant association. This analysis indicated that our study had 87.8% power to detect increases in firearm injuries as large as or larger than those seen for Nevada shows and 84.2% power to detect the difference between California and Nevada gun shows.

Appendix Figure 1.

Comparison of observed and simulated distribution of firearm deaths and injuries during the 2 weeks after gun shows in nearby regions.

The unit of analysis is the gun show. The figure presents the distribution of the observed and simulated number of firearm deaths and injuries, per gun show, ≤2 wk after each gun show, in regions within driving distance of each show, by state. For example, in both the observed and simulated data, just fewer than 300 gun show regions had no firearm deaths or injuries ≤2 wk after the show.

Bias Analysis

To assess the potential role of residual confounding due to unmeasured factors, we did a quantitative bias analysis. We estimated the characteristics of an unmeasured confounder that would yield the observed associations between gun shows and firearm injuries, if the true effect were not statistically significant.

Role of the Funding Source

This research was funded by the Eunice Kennedy Shriver National Institute of Child Health and Human Development; the National Institutes of Health Office of the Director; the University of California, Berkeley, Committee on Research; and the Heising-Simons Foundation. The funding sources had no role in study design, conduct, data collection, data analysis, preparation of the manuscript, or the decision to submit the manuscript for publication.

Results

We identified 640 gun shows in California and 275 in Nevada between 1 January 2005 and 31 December 2013 (Table 1). Shows were held on weekends, usually at convention centers or county fairgrounds, and lasted 2 to 3 days. Some shows returned to the same locations at regular intervals, whereas others were held at irregular times and locations. Overlap exclusions were more common for Nevada shows than for California shows because those in Nevada were more frequent and held in fewer locations. Appendix Figure 2 is a map of gun show locations. Table 1 provides characteristics of California and Nevada gun shows and total person-weeks of exposure to gun shows.

Table 1. Characteristics of California and Nevada Gun Shows and Population Exposure to Gun Shows

Appendix Figure 2.

Locations of gun shows in California and Nevada.

Table 2 presents the number and rate of local firearm injuries before and after California and Nevada gun shows. In the 2 weeks preceding California shows, 15 000 firearm injuries occurred in at-risk regions, but before–after rates remained stable for California shows. For Nevada shows, only 44 firearm injuries occurred during the preshow period (Table 2). However, firearm injury rates increased from 0.67 injuries (95% CI, 0.55 to 0.80 injuries) per 100 000 persons to 1.14 injuries (CI, 0.97 to 1.30 injuries) per 100 000 persons in California regions exposed to Nevada shows.

Table 2. Unadjusted Analyses of the Association Between Firearm Deaths and Injuries in California and Gun Shows in California and Nevada

After adjustment, California shows were not associated with increases in firearm injuries (RR, 0.99 [CI, 0.97 to 1.02]), but Nevada shows were associated with significant cross-border increases in firearm injuries in California (RR, 1.69 [CI, 1.16 to 2.45]) (Table 3). The difference between states was significant: Gun shows in Nevada were associated with a 70% greater increase in firearm injuries than those in California (ratio of RRs, 1.70 [CI, 1.17 to 2.47]). This association corresponds to a rate difference of 0.46 (CI, 0.36 to 0.57) per 100 000 persons, or a 0.3-SD increase relative to the biweekly variability in rates across locations. In terms of cases, the association corresponds to 30 additional injuries in regions exposed to the 161 Nevada shows.

Table 3. Adjusted Analyses of the Association Between Firearm Deaths and Injuries in California and Gun Shows in California Versus Nevada

The association with Nevada shows was driven by significant increases in firearm injuries from interpersonal violence (RR, 2.23 [CI, 1.01 to 4.89]). Results for analyses stratified by gun show state (Table 4) or restricted to regions similar to those exposed to Nevada shows (Table 5) were consistent with those from the main analysis. No significant relationships existed between gun shows and firearm injuries along known trafficking routes or when excluding California's 10-day waiting period (Appendix Table 2).

Table 4. Adjusted Analyses of the Association Between Firearm Deaths and Injuries in California and Gun Shows in California and Nevada, by State

Table 5. Adjusted Analyses of the Association Between Firearm Deaths and Injuries in California and Gun Shows in California Versus Nevada, Restricted to Regions Similar to Those Exposed to Nevada Gun Shows

Table 5. Adjusted Analyses of the Association Between Firearm Deaths and Injuries in California and Gun Shows in California Versus Nevada, Restricted to Regions Similar to Those Exposed to Nevada Gun Shows

Appendix Table 2. Secondary Analyses for the Adjusted Association Between Firearm Deaths and Injuries Along Firearm Trafficking Routes and Excluding California's 10-Day Waiting Period

In sensitivity analyses of geographic range and duration of exposure (Appendix Table 3), associations between California shows and firearm injuries were consistently null. For Nevada gun shows, changes in firearm injuries remained statistically significant for shorter (1-week) and longer (3-week) periods but were not statistically significant for smaller geographic ranges (60-minute drive), which yielded very few cases, or larger geographic ranges (120- and 180-minute drives for California and Nevada guns shows, respectively), which covered large portions of California. Nevada shows were significantly associated with increases in self-directed intentional firearm injuries when longer periods were examined.

Appendix Table 3. Sensitivity Analyses for the Adjusted Association Between Firearm Deaths and Injuries in California and Gun Shows in California Versus Nevada

The Appendix provides bias and negative control analyses. In brief, for the association between Nevada gun shows and California firearm injuries to be spurious, another factor would have to match the precise geographic and temporal pattern of the 275 Nevada gun shows and also be strongly associated with firearm injuries in California, corresponding to RRs of at least 1.5 or 2. This factor would also have to be up to 80% more prevalent in the 2 weeks after Nevada gun shows than in the 2 weeks before. Bias analysis results were similar for the difference-in-differences estimate comparing Nevada with California. Associations between both California and Nevada gun shows and nonfirearm injuries were null, or were statistically significant because of the large number of nonfirearm unintentional injuries (n = 6 065 633) but not meaningfully different from the null.

Discussion

We examined the association between California and Nevada gun shows and short-term changes in local firearm injuries in California. Using a quasi-experimental, difference-in-differences design, we took advantage of natural variation in the timing and location of gun shows and differences between California and Nevada firearm regulations to compare this association by state. Firearm injuries in California remained stable after California gun shows but increased by a small but significant amount after Nevada shows.

Several factors could explain our findings. First, although we did not formally assess the effect or enforcement of firearm policies in either state, the absence of an increase in firearm injuries after California gun shows may be evidence that California's strict regulatory environment, both gun show–related and otherwise, mitigates potential risk from gun shows through deterrence. Among other restrictions, California requires that all private transfers be documented by a licensed dealer and include a background check (22). It also enforces restrictions on gun shows (23), including a range of security- and enforcement-related planning and reporting practices, that may deter the illegal firearm activity historically seen at gun shows (15, 17, 18, 20). Specialized firearm enforcement agents from the California Department of Justice also do surveillance at California gun shows. In contrast, Nevada does not require background checks or documentation for private transfers and places no regulations on gun shows. Thus, California's measures may prevent illegal activities that could lead to increases in interpersonal firearm injuries.

A second possibility is that California's regulations and 10-day waiting period motivate buyers to cross into Nevada when seeking a faster, less-regulated source of firearms. This mechanism, which suggests displacement rather than deterrence, would imply that even if California's regulations are mitigating risk from gun shows within its borders, travel to less-restrictive states may threaten the effectiveness of California's laws. Indeed, interstate gun trafficking, including that between Nevada and California, is well-documented and fueled in part by gun shows (18–21, 38, 39).

A third possibility is that gun shows affect Californians near Nevada differently from Californians in the rest of the state. However, analyses restricted to regions similar to those along the California–Nevada border produced results consistent with the main analysis, suggesting that the characteristics of border communities are not major drivers of the observed differences.

Gun show occurrence was not randomized. Thus, a fourth possibility is that the observed association is due to uncontrolled confounders. However, gun shows were not associated with nonfirearm injuries, providing evidence that the results are not due to confounding by common causes of firearm and nonfirearm injuries (34). Furthermore, the quantitative bias analysis indicated that for the observed associations to be spurious, at least 1 factor would have to match the geographic and temporal pattern of the gun shows, be strongly associated with firearm injuries, be unevenly distributed between California and Nevada, and change markedly in prevalence in the 2 weeks after gun shows compared with the 2 weeks before. Identifying a factor that fits these criteria is challenging, which strengthens confidence in our results. Similar bias analyses have been used to bolster evidence of the association between firearm ownership and suicide (40).

Our null findings for California gun shows are consistent with those of Duggan and colleagues (25). However, our study was the first to our knowledge to assess interstate associations and suggests that travel across state lines may be important. Our study avoided several limitations highlighted in previous critiques of Duggan and colleagues' study (26, 27) by being well-powered statistically, analyzing data from the show-period level rather than the ZIP code–week level, and accounting for California's 10-day waiting period. Our approach was also strengthened by inclusion of nonfatal injuries and unintentional and intent-undetermined firearm injuries, rather than only firearm suicides and homicides. In addition, we examined geographic areas defined by driving distances and incorporated overlap exclusions for regions simultaneously exposed to 1 show and unexposed to another.

This study had limitations. First, all nonexperimental studies are subject to residual confounding. We minimized the effect of potential confounders by comparing identical regions over brief periods, during which factors other than gun shows are unlikely to vary; we also did negative control and quantitative bias analyses to assess the sensitivity of our results to an unobserved confounder. Second, few firearm injuries occurred in regions exposed to Nevada gun shows; however, rates for this region were derived from 13 037 052 person-weeks of exposure (Table 1). Third, cause-of-death and cause-of-injury classification on death and discharge records is imperfect, although studies suggest that the degree of misclassification is not substantial enough to alter major trends and patterns (41, 42). Fourth, we did not examine associations with firearm injuries in Nevada populations. Future research on the effects of gun shows in Nevada and other states would be valuable. Fifth, data on nonfatal injuries include most hospital visits for firearm injuries but do not include military hospitals. Lastly, evidence suggests that firearms purchased at gun shows and recovered from crime scenes are rarely found in the immediate region or period after shows (27). However, these patterns do not preclude the possibility of a proximate effect, particularly because first use of a gun may predate its recovery from a crime scene.

In conclusion, gun shows are an important source of firearms and may offer an opportunity for regulatory intervention. Results from this study suggest that California gun shows are not associated with short-term increases in firearm injuries but that Nevada shows are associated with cross-border increases in firearm injuries in California. Differences in regulations may explain this pattern, but alternative explanations exist, and the short-term increase in firearm injuries attributable to gun shows is small relative to the number of firearm injuries in places exposed to gun shows. Better understanding the long-term effects of gun shows over larger geographic regions, the effects of gun show policies, and the patterns of acquisition and use of firearms would provide important evidence to inform future efforts to prevent firearm injuries.

Appendix: In-State and Interstate Associations Between Gun Shows and Firearm Deaths and Injuries: Supplemental Information, Methods, and Results

Firearm Injury Classification Codes

We identified fatal and nonfatal firearm injuries in California between 2005 and 2013 using International Classification of Diseases, 9th and 10th Revisions, external cause-of-injury codes contained in death records from the California Department of Public Health Vital Records and in emergency department and inpatient hospitalization records from the Office of Statewide Health Planning and Development. To examine variation by firearm injury type, we did subgroup analyses for intentional interpersonal violence, intentional self-harm, unintentional injuries, and injuries of undetermined intent. International Classification of Diseases codes used in this analysis are presented by subgroup in Appendix Table 1.