Systematic Review of the Empirical Evidence of Spatial Displacement and Diffusion of Benefit among Geographically Focused Policing Initiatives Dr Shane D Johnson, Dr Kate Bowers, Dr Rob Guerrette, Lucia Summers and Dr Suzanne Poynton Department of Security and Crime Science University College London (UCL) 2010 International NPIA-Cambridge Conference on Evidence-Based Policing
Overview • Background: The need for a review • Methods: • Inclusion criteria • Search strategy • Study coding • Analytic methods • Results • Authors’ effect sizes • Simple proportions • Meta-analysis
Background • Criticisms that focused policing efforts do not address the “root causes” of crime • Displacement is the relocation of crime from one place, time, target, offence, tactic or offender to another as a result of some crime prevention initiative • Of the six possible types, spatial displacement is the form most commonly recognised (Eck 1993) • At the extreme, widespread displacement stands to undermine the effects of geographically focused policing actions Eck, J.E. (1993). The threat of crime displacement. Criminal Justice Abstracts, 253:527-546.
Background (cont.) • Research suggests that crime displacement is rarely total • At the other end of the displacement continuum is the phenomenon of diffusion of crime control benefits • Two (or more) mechanisms for diffusion (Clarke and Weisburd 1994): • deterrence: elevated risk of detection and arrest • discouragement: effort exceeds anticipated rewards • Police and others often assume a homogeneous group of motivated offenders Clarke, R.V. and Weisburd, D. (1994). Diffusion of crime control benefits: Observations on the reverse of displacement. In R.V. Clarke (Ed.), Crime Prevention Studies, Vol. 2. Monsey, NY: Criminal Justice Press.
The need for a review • Experiments on the extent of displacement and diffusion following focused policing efforts, but no systematic appraisal • Related reviews: • Barr and Pease (1990) • Eck (1993) • Hesseling (1994) • Guerette and Bowers (2009) Barr, R. and Pease, K. (1990). Crime placement, displacement and deflection. In M. Tonry and N. Morris (Eds.), Crime and Justice: A review of research, Vol. 12. Chicago: University of Chicago Press. Hesseling, R. (1994). Displacement: A review of the empirical literature. In R.V. Clarke (Ed.), Crime Prevention Studies, Vol. 3. Monsey, NY: Criminal Justice Press. Guerette, R.T. and Bowers, K. (2009). Assessing the extent of crime displacement and diffusion of benefits: A review of situational crime prevention evaluations. Criminology, 47(4): 1331-1368.
METHOD - Paper inclusion criteria • Study must evaluate a focused policing intervention • hotspot policing/ directed patrol • police crackdown • problem-oriented/ intelligence-led policing project • community policing intervention • broken windows/ Compstat approaches • civil injunctions/ civil remedy • police-led environmental improvement • Intervention was ‘geographically focused’ to a local area • INCLUDED: Census blocks, police zones/beats/divisions/precincts, estates, districts, suburbs, block areas, series of roads, neighbourhoods • EXCLUDED: Very large scale (e.g. entire city)
METHOD - Paper inclusion criteria (cont.) • Quantitative measure of crime • for the ‘treatment’, ‘control’ and ‘displacement/diffusion catchment’ areas • pre- and post-intervention (or pre- and during) • those without a control area were considered BUT not included in the meta-analysis • Study written in English • Paper reported original research findings • no meta-analyses or reviews • if multiple papers per study, the most detailed was used • Any point in time and any location • Both published and unpublished studies
METHOD – Search strategy • Keyword search of electronic abstract databases (displac* OR “diffusion of benefit” OR “diffusion of benefits” OR “multiplier effect” OR “free side benefit” OR “ halo effect” OR “spill over*” OR “free rider effect” OR “bonus effect” OR “spill-over”) AND (police OR policing OR law enforcement) AND (“hot spot policing” OR ‘hot spots policing” OR crackdown* OR “problem oriented policing” OR “problem solving” OR “focused policing” OR “targeted policing” OR “directed patrol” OR “enforcement swamping” OR “intelligence led policing” OR “broken windows” OR “compstat” OR “community policing”) AND (evaluat* OR impact OR assessment OR test)
METHOD – Search strategy (cont.) • Bibliography search of: • existing displacement reviews (Barr and Pease 1990; Eck 1993; Hesseling 1994; Guerette and Bowers 2009); and • reviews of the effectiveness of focused policing initiatives (Braga 2007; Mazerolle et al. 2007; Weisburd et al. 2008). Braga, A.A. (2007). Effects of Hot Spots Policing on Crime. Campbell Collaboration systematic review, available from www.campbellcollaboration.org Mazerolle, L.; Rombouts, S. and Soole, D.W. (2007). Street-level Drug Law Enforcement: A meta-analytic review. Campbell Collaboration systematic review, available from www.campbellcollaboration.org Weisburd, D.; Telep, C.W.; Hinkle, J.C. and Eck, J.E. (2008). The effects of problem-oriented policing on crime and disorder. Campbell Collaboration systematic review, available from www.campbellcollaboration.org
METHOD – Search strategy (cont.) • Forward search for works that have cited key displacement publications (Bowers and Johnson 2003; Clarke 1994; Clarke and Weisburd 1994; Weisburd et al. 2006) • A review of research reports of professional research and policing organisations • Hand search of pertinent journals Bowers, K. and Johnson, S.D. (2003). Measuring the geographical displacement and diffusion of benefit effects of crime prevention activity. Journal of Quantitative Criminology, 193:275-301. Clarke, R.V. (1994). Displacement: An old problem in new perspective. In G. Saville (Ed.), Crime Problems, Community Solutions: Environmental criminology as a developing prevention strategy. Port Moody, British Columbia: AAG Inc. Publications. Weisburd, D.; Wyckoff, L.; Ready, J.; Eck, J.; Hinkle, J.C. andGajewski, F. (2006). Does crime just move around the corner? a controlled study of spatial displacement and diffusion of crime control benefits. Criminology, 443: 549-591.
Analytical strategy • Proportional change analysis • Summary of effect sizes (ESs) as reported by the study authors • Meta-analysis • Weighted Displacement Quotient (WDQ) [not covered here]
2. Summary of authors’ effect sizes • 19 studies reported statistical test results • Displacement findings: • 15: no significant increases in treatment catchment area(s) • 2: significant increases but intervention not effective • 2: significant increases for some crime types (1 of 10; 1 of 2) • Diffusion of benefit findings: • 8: no significant decreases in treatment catchment area(s) • 7: significant decreases but only for some crime types, some of the contiguous areas or when using certain tests AND/OR when intervention not effective • 2: significant decreases in treatment catchment area(s) • 2: inconclusive
3. Meta-analysis • Pre- and post-intervention counts of crime commonly reported • Sometimes counts not given – BUT in most cases figures could be converted • Odds Ratio (OR) calculations used to estimate ES and CIs for BOTH treatment area and catchment area (Only possible where numbers are available for a suitable control area) • Random effects model used for mean ES (Many studies have more than one observation for the same treatment/control/catchment area)
Conclusions Results suggest that, on average, geographically focused policing initiatives for which data were available were: • associated with significant reductions in crime and disorder • overall, changes in (immediate) catchment areas are non-significant but there is a trend in favour of a diffusion of benefit • for RCTs, there is a diffusion of benefit and the mean effect is statistically significant More in the review: • WDQ analysis • Differences by intervention type, size of treatment area, etc. • Discussion of methodological issues
Thank you for your attentionLucia Summersl.firstname.lastname@example.org 2010 International NPIA-Cambridge Conference on Evidence-Based Policing