In 2013, the Chicago Police Department conducted a pilot of a predictive policing program designed to reduce gun violence. The program included development of a Strategic Subjects List (SSL) of people estimated to be at highest risk of gun violence who were then referred to local police commanders for a preventive intervention. The purpose of this study is to identify the impact of the pilot on individual- and city-level gun violence, and to test possible drivers of results.
The SSL consisted of 426 people estimated to be at highest risk of gun violence. We used ARIMA models to estimate impacts on city-level homicide trends, and propensity score matching to estimate the effects of being placed on the list on five measures related to gun violence. A mediation analysis and interviews with police leadership and COMPSTAT meeting observations help understand what is driving results.
Individuals on the SSL are not more or less likely to become a victim of a homicide or shooting than the comparison group, and this is further supported by city-level analysis. The treated group is more likely to be arrested for a shooting.
It is not clear how the predictions should be used in the field. One potential reason why being placed on the list resulted in an increased chance of being arrested for a shooting is that some officers may have used the list as leads to closing shooting cases. The results provide for a discussion about the future of individual-based predictive policing programs.
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This is only one component of the larger long-term collaboration which broadly explores whether and how crime can be predicted.
Here, “numerical instability” refers to the likelihood that the estimates that an SSL member’s calculated risk of a specified thousands of times more likely to be killed is due more to statistical artifacts from fitting the quadratic curve than an accurate estimate.
Initially, CPD said they would put all the highest-risk individuals on the list; however, they decided to vet the list through their Deployment Operations Center (DOC), who made some changes to who would appear on the list, and therefore, the 426 individuals did not represent the highest scoring individuals based on the model.
Details of GVRS are available at http://directives.chicagopolice.org/directives/data/a7a57bf0-136d1d31-16513-6d1d-382b311ddf65fd3a.html
It is important to note that Chicago has gone through transformative changes over this time period, including a new Superintendent in 2011 and the integration of COMPSTAT to provide oversight. In addition to changes in leadership and management style, CPD has implemented a large number of homicide reduction strategies during this time period, including the multiple changes to the Gang Violence Reduction Strategy, and gang call-ins across different districts starting in 2010.
While there is always the possibility that the groups are different on unobservable variables, we have captured many of the important research-validated criminogenic factors. That is why we specify the approach reduces, rather than eliminates, bias.
With the exception of the winter of 2012 which did not experience the same degree of a “cooling off” period.
Most arrestees were not incapacitated for any significant period of time, but rather were booked into the Cook County jail and released within a few hours to a few days.
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We would like to thank the Chicago Police Department and Dr. Miles Wernick from the Illinois Institute of Technology for their participation and support of this evaluation. We would also like to acknowledge research assistance provided by Sam Cooper and Alessandra Sienra-Canas. This publication was made possible by Award Number 2009-IJ-CX-K114 - Predictive Policing Analytic & Evaluation Research Support awarded by the National Institute of Justice, Office of Justice Programs. The opinions, findings, conclusions and recommendations expressed in this publication are those of the authors and do not necessarily reflect the views of the Department of Justice.
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Saunders, J., Hunt, P. & Hollywood, J.S. Predictions put into practice: a quasi-experimental evaluation of Chicago’s predictive policing pilot. J Exp Criminol 12, 347–371 (2016). https://doi.org/10.1007/s11292-016-9272-0