Eric L. Piza, Rachael A. Arietti, Jeremy G. Carter, and George O. Mohler (2023)

Journal of Experimental Criminology

*This study was funded by the National Institute of Justice (grant number 2019-R2-CX0004)

Key Takeaways

• This study tests whether (1) shots fired calls for service in the gunshot detection technology (GDT) target area are more likely to be classified as unfounded; (2) police responses to shootings in the GDT target area are more likely to recover ballistic evidence or firearms; and (3) shootings in the GDT target area are more likely to be cleared
• Shots fired calls for service in the GDT target area were 15% more likely to be unfounded
• For fatal shootings, GDT treatment was not associated with increased likelihood of ballistic evidence collection or case clearance
• For non-fatal shootings, GDT treatment was not associated with increased likelihood of ballistic evidence collection, gun recovery, or case clearance
• GDT may not add value to investigations and may increase patrol workload

Research Summary

Clearance rates have long been used as a measure of police performance and effectiveness, reflecting the importance of incapacitation in disrupting patterns of violence and creating a general deterrence effect, and the public desire to deliver justice to crime victims and their families. Research relating to the level to which technology improves case clearance are highly mixed, with technology improving investigative function in certain cases but not others.

Gunshot detection technology (GDT) has become a central component of police efforts to respond to and investigate gun violence. Over 250 public safety agencies worldwide have adopted the ShotSpotter platform developed by SoundThinking, the global industry leader in GDT. The majority of evaluation studies have explored GDT’s crime prevention capacity, despite the technology arguably offering more potential for investigative police functions. The few studies that have focused on GDT’s impact on investigative outcomes, such as evidence collection and case clearance, have generated little consensus.

The current study evaluates GDT’s effect on evidence collection and case clearance in Kansas City, MO. The Kansas City Police Department (KCPD) installed SoundThinking’s ShotSpotter GDT system in September 2012, with the target area covering approximately 3.5 square miles of the city. Kansas City pays between $227,500 and $315,000 per year for their ShotSpotter system based on the advertised annual subscription cost of between $65K and $90K per square mile. The system detected 11,517 gunfire events through the end of 2019, the final year of our study period.

We used the entropy balancing method to conduct a matched case–control evaluation. Entropy balancing is a quasi-experimental design that matches treatment and control units by reweighting covariates based on propensity for treatment. The sum of the control unit weights equals the total number of cases in the treated group. Eighteen covariates were used in the entropy matching algorithm to create a weighted control group that mirrors the treatment group across relevant measures. The influence of GDT was tested through logistic regression models incorporating the weights from the entropy matching procedure. The entropy weights were incorporated as probability weights in the logistic regression models.

Results indicate that shots fired calls for service in the GDT target area have a 15% increased likelihood of being unfounded as compared to the control group. Fatal shooting incidents in the GDT target area were no more likely to result in collection of ballistic evidence for NIBIN analysis, or subsequent case clearance, as compared to incidents in the control area. Non-fatal shooting incidents in the GDT target area were no more likely to result in collection of ballistic evidence for NIBIN analysis, the recovery of firearms, or subsequent case clearance, as compared to incidents in the control area.

The current study did not find support for GDT as an investigatory tool for either fatal or non-fatal shooting incidents. The increased likelihood of unfounded shots fired cases has important implications for GDT use by police. This reflects increased workload of police responding to incidents where gunfire was not confirmed.

Eric L. Piza, David N. Hatten, George O. Mohler, Jeremy G. Carter, and Jisoo Cho (2023)

Criminology & Public Policy

*This study was funded by the National Institute of Justice (grant number 2019-R2-CX0004)

Key Takeaways

• Gunshot detection technology (GDT) was associated with ~30% higher levels of ballistic evidence collection in the GDT target area and surrounding catchment area
• GDT was associated with ~30% higher levels of gun recoveries in the surrounding catchment area
• GDT was associated with ~22% lower levels of shots fired calls for service in the GDT target area
• GDT did not influence any of the gun violence categories involving confirmed victims (non-fatal shootings, fatal-shootings, and aggravated assaults/robberies committed with a firearm)
• Agencies that prioritize gun violence reduction—rather than reducing calls for shots fired or increasing ballistic evidence collection—should consider if resources are better used for solutions other than GDT

Research Summary

Gunshot detection technology (GDT) has recently emerged as a core entry into the suite of technological gun violence prevention solutions incorporated by police. Despite increased popularity of the technology, the research evidence on GDT is underdeveloped, especially as compared to other police technologies. While certain GDT studies have taken efforts to select control areas with similar crime and sociodemographic conditions as the target areas, this is not commonplace in GDT research. Furthermore, such research has used a fuzzy matching approach where control areas are selected based on their general similarity with target areas rather than through quantitative matching techniques that ensure statistical equivalency between treatment and control areas.

The current study aims to contribute to the knowledge on GDT effect on crime occurrence through a rigorous evaluation of the technology in Kansas City, MO. The Kansas City Police Department installed SoundThinking’s ShotSpotter GDT system in September 2012, with the target area covering approximately 3.5 square miles of the city. Kansas City pays between $227,500 and $315,000 per year for their ShotSpotter system based on the advertised annual subscription cost of between $65K and $90K per square mile. The system detected 11,517 gunfire events through the end of 2019, the final year of our study period.

We apply the recently developed microsynthetic control method in the evaluation, incorporating over 13 years of data. The microsynthetic control method modifies the synthetic control method for application to micro-geographic units of analysis. The control group was specified to match the GDT target area across 18 covariates.

Both process and outcome measures were tested in the analysis. Process measures included gun recoveries and NIBIN ballistic evidence collection to reflect the enforcement-related causal mechanisms of GDT. Outcome measures included shots fired calls for service, non-fatal shootings, fatal shootings, and gun assaults and robberies. The statistical analysis was conducted for both the GDT target area to measure main effects and a surrounding catchment area to measure spatial displacement effects.

The collection of NIBIN ballistic evidence was significantly higher by approximately 30% compared to the weighted control area in both the GDT target area (476 vs. 365) and catchment area (351 vs. 271). Gun recoveries were nearly 13% higher in the catchment area than the weighted control area (1,668 vs. 1,477). Shots fired calls for service were approximately 22% lower in the GDT target area than the weighted control area (5,665 vs. 7,285). Importantly, none of the three crime types involving confirmed victims (fatal shootings, non-fatal shootings, gun assaults and robbery) exhibited any significant changes following the installation of GDT in either the target area or catchment area.

Overall, the study results do not offer much empirical support for GDT as a gun violence prevention tool in Kansas City. Agencies that highly prioritize increasing evidence collection and reducing unauthorized firearm discharges may consider dedicating necessary resources to acquire GDT. Agencies that prioritize the reduction of gun violence victimization, however, should consider whether resources are better used for solutions other than GDT.