New preprint: The accuracy of the violent offender identification directive (VOID) tool to predict future gun violence

I have a new preprint out, The accuracy of the violent offender identification directive (VOID) tool to predict future gun violence. This is work with Rob Worden and Jasmine Silver from our time at the Finn Institute. Below is the abstract:

We evaluate the Violent Offender Identification Directive (VOID) tool, a risk assessment instrument implemented within a police department to prospectively identify offenders likely to be involved with future gun violence. The tool uses a variety of static measures of prior criminal history that are readily available in police records management systems. The VOID tool is assessed for predictive accuracy by taking a historical sample and calculating scores for over 200,000 individuals known to the police at the end of 2012, and predicting 103 individuals involved with gun violence (either as a shooter or a victim) during 2013. Despite weights for the instrument being determined in an ad-hoc manner by crime analysts, the VOID tool does very well in predicting involvement with gun violence compared to an optimized logistic regression and generalized boosted models. We discuss theoretical reasons why such ad-hoc instruments are likely to perform well in identifying chronic offenders for all police departments.

There were just slightly over 100 violent gun offenders we were trying to pick out of over 200,000. The VOID tool did really well! Here is a graph comparing how many of those offenders VOID captured compared to a generalized boosted model (GBM), and two different logistic regression equations.

I have some of my thoughts in this article as to why a simple tool does just as well as more complicated regression and machine learning techniques, which is a common finding in recidivism studies as well. My elevator pitch for why that is is because most offenders are generalists, and for example you can basically swap prior arrests for robbery with prior arrests for motor vehicle theft — they both provide essentially the same signal for future potential criminality. See also discussion of this on Dan Simpson’s post on the Stat Modeling, Causal Inference and Social Science blog, which in turn makes me think the idea behind simple models can be readily applied to many decision points in the criminal justice field.

The simple takeaway from this for crime analysts making chronic offender lists is that don’t let the perfect be the enemy of the good. Analysts can likely create an ad-hoc weighting to prioritize chronic offenders and it will do quite well compared to fancier models.

I will be presenting this work at the ACJS conference in New Orleans on Saturday 2/17/18. It is a great session, with YongJei Lee, Jerry Ratcliffe, Bryanna Fox, and Stacy Sechrist (see session 384 in the ACJS program), so stop on by. If you want to catch up with me in New Orleans just send me an email. And as always if you have feedback on the draft I am all ears.