The Shot Spotter Pretext - Part 2: Chicago Inspector General Report Findings Show Shot Spotter Not Effective Tool


On Monday, July 26th, 2021, Chicago-One published part one of a series on Chicago's Shotspotter technology, called, "The Shot Spotter Pretext"

Today, the city's Inspector General released a 30-page report that made evidence-based findings about Shotspotter technology usage in the city by Chicago Police. 

The city OIG stated in their report:  

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.

    The report also stated, "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."

    The report further stated "The City’s three-year contract with ShotSpotter began on August 20, 2018, through August 19, 2021, at a cost of $33 million. In November 2020, well before the end of the contract term, CPD requested an extension of the contract and in December 2020, the City exercised an option to extend it, setting a new expiration date for August 19, 2023. In March 2021, CPD requested approval for an annual 5% increase in the cost per square mile of the contract.

    The city Inspector General's Office also raised another concern: 

    "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."

    How Shotspotter works

    "The ShotSpotter system approximates the location of the possible gunshots via triangulation and multilateration—two techniques for computing the source location of a sound based on the time of arrival and angle of arrival of sound waves at multiple surrounding sensors." - Chicago OIG ShotSpotter report, Shotspotter PDF file. 

    According to today's Chicago OIG report, "In 2014, ShotSpotter produced a white paper that explains, in detail, the technical science behind the acoustic
    gunshot detection technology, including a discussion of false positive alerts. ShotSpotter, “Gunshot Detection Technology,” September 2, 2014, accessed June 14, 2021."

    Graphic from the August 24th, 2021 Chicago Inspector General report showing how ShotSpotter works

The Shotspotter technology covers roughly or about half of Chicago's 22 police districts across 5 regional Chicago Police Detective Divisions. ShotSpotter sensors have been installed in CPD’s 2nd, 3rd, 4th, 5th, 6th, 7th, 8th, 9th, 10th, 11th, 15th, and 25th Districts, per the OIG report. 

What Happens When Police Receive a ShotSpotter Notification?

The Chicago OIG report states the following:

ShotSpotter is one of the tools used by analysts in CPD’s Strategic Decision Support Centers (SDSCs), CPD District-based centers that are “equipped with crime-reduction tools and technology to assist [CPD] members with district-crime forecasting and achieving the primary mission of district crime-reduction.”

The first SDSCs were established in 2017 in partnership with the University of Chicago Crime Lab. After a shots fired incident is detected and confirmed by a ShotSpotter-employed “acoustic expert” at a ShotSpotter office, the alert is displayed on the ShotSpotter application, which is accessible by the CPD members assigned to the SDSC (“SDSC analysts”), OEMC, and CPD members who are equipped with the ShotSpotter mobile application on CPD-issued smartphones. SDSC analysts monitor the ShotSpotter application for these incoming alerts. When alerts come through, pursuant to CPD directives, SDSC analysts are responsible for initiating the dispatch process by contacting OEMC to report the ShotSpotter alert. OEMC personnel will then issue an event number—a unique identification number assigned to every distinct incident of police activity—for a ShotSpotter alert and dispatch CPD units to respond.

SDSCs were initially staffed by analysts who were employed by the University of Chicago Crime Lab. In January 2020, after a transitional phase during which CPD hired analysts and the University of Chicago Crime Lab trained them, the University of Chicago Crime Lab analysts moved out of the CPD District SDSCs. University of Chicago Crime Lab, “Strategic Decision Support Centers (SDSCs),” accessed June 14, 2021. 

At the conclusion of any law enforcement activity, the primary responding CPD unit is to report a disposition—the outcome of the incident—to OEMC. OEMC will then record the corresponding disposition code in the record for the event number. Criminal incidents are assigned an Illinois
Uniform Crime Reporting code.

Incidents that are not criminal in nature but require the completion of a case report, such as a traffic crash, are assigned a non-criminal incident code.

For incidents that do not require the completion of a case report, CPD also defines a set of “miscellaneous incident” disposition codes. When a CPD member responds to a ShotSpotter alert, they are to take investigative steps which may include interviewing witnesses, conducting investigatory stops, running license plates, searching for shell casings, etc. If these activities
produce evidence of a shooting or any other criminal activity, a corresponding criminal incident code will be assigned. If there is no such evidence, then the event will receive a miscellaneous incident disposition code.

According to the City of Chicago Office of Public Safety Administration, crime classification codes are “derived from the Federal Bureau of Investigation's (FBI) National Incident-Based Reporting System (NIBRS) Uniform Crime Reporting (UCR) Program.” NIBRS codes report the crime type and UCR codes report the specific criminal offense. City of Chicago Office of Public Safety Administration, “Definition & Description of Crime Types”.

The Chicago Office of The Inspector General released details of how they pulled and matched data from ShotSpotter to CPD ISR reports, and also released data related to ShotSpotter hits by district and beat. 

The next data set released today with this report shows data for SHOTSPOTTER ALERTS: INCIDENT DISPOSITIONS

In 8,346 of the 50,176 confirmed ShotSpotter alerts (16.6%), no disposition code indicating the final outcome of the event was recorded in the OEMC event record. The remaining 41,830 ShotSpotter alerts reported either a criminal incident disposition, a non-criminal incident disposition, or a miscellaneous incident disposition. Criminal incident dispositions account for 13.2% of OEMC records that include disposition data, representing 5,504 criminal case reports completed following a CPD response to a ShotSpotter alert. Of that total, 4,556 criminal case reports—82.8% of criminal incident dispositions and 10.9% of all records reporting a disposition—listed charges which are likely related to gun violence or illegal gun possession.

Figure 4 below displays the monthly total of ShotSpotter alerts alongside the monthly total of ShotSpotter alerts which recorded likely-gun-related crime disposition.

Figure 5 displays law enforcement outcomes for the ShotSpotter alerts matched to ISRs via event number. The 1,056 matched event numbers indicate that investigatory stops are documented as associated with a specific ShotSpotter alert in only 2.1% of the 50,176 CPD dispatches to
ShotSpotter alerts.

“Enforcement actions” include issuance of citations and ordinance
violations in addition to arrests. According to the data collected in the associated ISRs, fewer than 2 in 10 investigatory stops following ShotSpotter alerts resulted in the recovery of a gun, with a high rate of 17.2% in the 11th District and a low of 4.7% in the 9th District.

A total of 1,366 investigatory stop reports (ISRs) completed between January 1, 2020 and May 31, 2021, include the keywords “SPOTTER” or “SST” but did not have an event number match to a confirmed ShotSpotter alert. 

From quantitative analysis of ShotSpotter data and other records, OIG concludes that CPD responses to ShotSpotter alerts rarely produce evidence of a gun-related crime, rarely give rise to investigatory stops, and even less frequently lead to the recovery of gun crime-related evidence during an investigatory stop.

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