A simple weighted displacement difference test to evaluate place based crime interventions

Andrew P Wheeler,Jerry H Ratcliffe

doi:10.1186/s40163-018-0085-5

Andrew P Wheeler, Jerry H Ratcliffe

Open Access

https://doi.org/10.1186/s40163-018-0085-5

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Journal: Crime Science	Publication Date: Sep 25, 2018
Citations: 14	License type: open-access

Affiliation: The University of Texas at Dallas, Temple University

Abstract

This work proposes a new statistic, the weighted displacement difference (WDD), as a simple test to evaluate crime reductions subsequent to place based interventions taking into account trends in comparison areas and potential spatial displacement or diffusion of benefits. The motivation is to have both a simple statistic that can give an estimate of the effectiveness of a crime reducing intervention, while also providing a measure of the standard error one would expect when examining crime counts simply by chance. The proposed statistic can be calculated in a spreadsheet with counts of crime before and after the intervention in both treated and displacement areas given matched control locations for each.Code to replicate the analysis as well as an Excel® spreadsheet to calculate the WDD and its standard error can be downloaded from https://www.dropbox.com/s/rg7x9bg1o3d8nsf/WDD_Analysis.zip?dl=0

Highlights

The weighted displacement quotient (WDQ) is a simple statistic to identify whether a place-based intervention reduced crime in a treatment area relative to a control area, while taking into account potential spatial displacement of crime (Bowers and Johnson 2003; Guerette 2009; Ratcliffe n.d.)
It has been used across a range of crime prevention evaluations, such as; burglary prevention (Bowers and Johnson 2003), estimating the impact of facilities such as public housing (Cahill 2011) or casinos (Johnson and Ratcliffe 2017) on crime rates, the crime prevention value of community support officers (Ariel et al 2016), the crime reduction effects of closing brothels (Soto and Summers 2018), and whether there were any impacts on localized property and violence crime levels after the introduction of CCTV cameras (Ratcliffe et al 2009)
There are four areas; the treated area T, a control area comparable to the treated area Ct, a displacement area D, and a second control area comparable to the displacement area Cd. These are indexed by counts of crime before the intervention took place with the subscript 0, and counts of crime in the post time period after the intervention took place with a subscript of 1. Note this is one more piece of additional information than the WDQ requires, a control area to correspond to the displacement area, which is sometimes considered in the evaluation literature but not frequently (Bowers et al 2011a; Telep et al 2014)

Summary

Introduction

The weighted displacement quotient (WDQ) is a simple statistic to identify whether a place-based intervention reduced crime in a treatment area relative to a control area, while taking into account potential spatial displacement of crime (Bowers and Johnson 2003; Guerette 2009; Ratcliffe n.d.). Bowers et al (2011a) have written briefly about using a simulation approach to accomplish this with the original WDQ, both the new WDD test statistic we propose here, and its standard error, can be calculated in a spreadsheet This statistic yields a simple tool for crime analysts to answer whether an intervention decreased crime, but to say whether that decrease was larger than one would expect, given that crime counts will always fluctuate up or down by some amount by chance

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