OIG Finds That ShotSpotter Alerts Rarely Lead to Evidence of a Gun-Related Crime and That Presence of the Technology Changes Police Behavior

The City of Chicago Office of Inspector General’s (OIG) Public Safety section has issued a report on the Chicago Police Department’s (CPD) use of ShotSpotter acoustic gunshot detection technology and CPD’s response to ShotSpotter alert notifications. OIG concluded from its analysis that CPD responses to ShotSpotter alerts can seldom be shown to lead to investigatory stops which might have investigative value and rarely produce evidence of a gun-related crime. Additionally, OIG identified evidence that the introduction of ShotSpotter technology in Chicago has changed the way some CPD members perceive and interact with individuals present in areas where ShotSpotter alerts are frequent.

OIG issued its descriptive analysis on the outcomes of ShotSpotter alerts to provide the public and City government officials—to the extent feasible given the quality of the Office of Emergency Management (OEMC) and CPD’s data—with clear and accurate information regarding CPD’s use of ShotSpotter technology. ShotSpotter is a gunshot detection system that uses a network of acoustic sensors to identify and locate suspected gunshots, and currently operates in more than 100 U.S. cities. Chicago’s $33 million, three-year contract with ShotSpotter began on August 20, 2018; in December 2020, well before the end of the contract term, the City exercised an option to extend the contract, setting a new expiration date for August 19, 2023.

The CPD data examined by OIG does not support a conclusion that ShotSpotter is an effective tool in developing evidence of gun-related crime. If this result is attributable in part to missing or non-matched records of investigatory stops that did take place as a direct consequence of a ShotSpotter alert, CPD’s record-keeping practices are obstructing a meaningful analysis of the effectiveness of the technology.

OIG analyzed data collected by CPD and OEMC regarding all ShotSpotter alert notifications that occurred between January 1, 2020, and May 31, 2021, and investigatory stops confirmed to be associated with CPD’s response to a ShotSpotter alert. OIG’s analysis of OEMC data and investigatory stop report (ISR) data revealed:

  • A total of 50,176 ShotSpotter alerts were confirmed as probable gunshots, issued an event number—a unique record identification number assigned to distinct “events” of police activity—and dispatched by OEMC; each of these resulted in a CPD response to the location.
  • Of the 50,176 confirmed, 41,830 report a disposition—the outcome of the police response to an incident. Of those dispositions, a total of 4,556 indicate that evidence of a gun-related criminal offense was found, representing 9.1% of CPD responses to ShotSpotter alerts.
  • Among the 50,176 confirmed and dispatched ShotSpotter alerts, a total of 1,056 share their event number with at least one ISR, indicating that a documented investigatory stop was a direct result of a particular ShotSpotter alert. That is, at least one investigatory stop is documented under a matching event number in 2.1% of all CPD responses to ShotSpotter alerts.
  • Through a separate keyword search analysis of all ISR narratives within the analysis period, OIG identified an additional 1,366 investigatory stops potentially associated with ShotSpotter alerts whose event number did not match any of the 50,176 confirmed and dispatched ShotSpotter alerts.

OIG’s analysis of ISR narratives further revealed that the presence of the technology is changing police behavior. Specifically, OIG reviewed instances in which CPD members rely, at least in part, on a perceived aggregate frequency of ShotSpotter alerts in an area to form the basis for an investigatory stop or as part of the rationale for a pat down once a stop has been initiated. Additionally, better data on law enforcement outcomes from ShotSpotter alerts would be valuable to support the City’s future assessments of whether to extend, amend, or discontinue its contractual relationship with ShotSpotter.

“Our study of ShotSpotter data is not about technological accuracy, it’s about operational value,” said Deputy Inspector General for Public Safety Deborah Witzburg. “If the Department is to continue to invest in technology which sends CPD members into potentially dangerous situations with little information––and about which there are important community concerns–– it should be able to demonstrate the benefit of its use in combatting violent crime. The data we analyzed plainly doesn’t do that. Meanwhile, the very presence of this technology is changing the way CPD members interact with members of Chicago’s communities. We hope that this analysis will equip stakeholders to make well-informed decisions about the ongoing use of ShotSpotter technology.”

The full report can be found online at OIG’s website.

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The mission of the independent and non-partisan City of Chicago Office of Inspector General (OIG) is to promote economy, effectiveness, efficiency, and integrity by identifying corruption, waste, and mismanagement in City government. OIG is a watchdog for the taxpayers of the City and has jurisdiction to conduct investigations and audits into most aspects of City government. If you see corruption, fraud, or waste of any kind, we need to hear from you. For more information, visit our website at: www.igchicago.org.