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How can Evidence Reviews make a difference to practice? Hot-spotting and displacement. Professor Kate Bowers Prof Shane Johnson, Dr Rob Guerrette, Dr Lucia Summers and Dr Suzanne Poynton Department of Security and Crime Science University College London (UCL). Overview.
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How can Evidence Reviews make a difference to practice? Hot-spotting and displacement Professor Kate Bowers Prof Shane Johnson, Dr Rob Guerrette, Dr Lucia Summers and Dr Suzanne Poynton Department of Security and Crime Science University College London (UCL)
Overview • How do we know whether geographical focused policing approaches prevent crime? • Available evidence • If it works how do we know that it doesn’t just move crime elsewhere? • Available evidence • Strengths and weakness of different approaches to evidence synthesis
Different Types of Evidence from Individual Evaluations The Maryland Scale for Evaluating Crime Prevention Source: Sherman et al. (1998) • Level 1: Correlation between a crime prevention programme and a measure of crime or crime risk factors at a single point in time • Level 2: Temporal sequence between the programme and the crime or risk outcome clearly observed, or the presence of a comparison group without demonstrated comparability to the treatment group. • Level 3: A comparison between two or more comparable units of analysis, one with and one without the programme. • Level 4: Comparison between multiple units with and without the programme, controlling for other factors, or using comparison units that evidence only minor differences. • Level 5: Random assignment and analysis of comparable units to programme and comparison groups
RCT in crime prevention: An exampleBraga, Anthony A., David L. Weisburd, Elin J. Waring, Lorraine Green Mazerolle, William Spelman, and Francis Gajewski. "Problem-Oriented Policing in Violent Crime Places: A Randomized Controlled Experiment." Criminology 37, no. 3 (August 1999). • RCT design • 56 high violence places (blocks) identified. • Matched into 28 pairs for evaluation purposes • Police selected 12 pairs for random allocation. • A coin was flipped to assign each in the pair to action/control • Treatment was a collection of specific problem-oriented tactics that could be broadly categorised as ‘policing disorder’ strategy. • Data source • Crime incident report data and citizen emergency calls for service • Analysis Model (GLM) log(count of crime events in post-test) = Intercept + (effect due to group) + (effect due to block) + log(count of crime events in pre-test) + error.
Synthesising the evaluations: Braga et al 2012 • Police-led efforts to control crime hotspots: • Directed patrol • Heightened traffic enforcement • Aggressive disorder enforcement • Problem-oriented policing • Outcome • Crime incident reports • Calls for service • Arrest data • 19 eligible studies • 89.5% USA • 73% Peer reviewed journals • 52.6% RCT designs
Mediators and moderators • Mediators are intermediate variables • E.g. X→Y→Z • They can help to give clues concerning the causal chain of events • A moderator refers to a context for efficacy variation • E.g G1: X→Y1; G2: X→Y2 • Moderators may refer to subgroups, e.g. sex, or settings, e.g. demonstration vs mainstream programmes • More and more studies are needed! • Mediator/moderator measurement foresight is needed • Currently in programmes for children, for example, ‘The use of moderators is sporadic and vague at best’ (Petrosino 2000, quoted in Pawson) • But still causal mechanisms, processes and programme modification/change are all overlooked
What about displacement/ diffusion? • Criticisms that focused policing efforts do not address the “root causes” of crime • Displacement is the relocation of crime from one place, time, target, offense, 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 recognized (Eck, 1993) • At the extreme, widespread displacement stands to undermine the effects of geographically focused policing actions
Background • Emerging 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 (Clarke and Weisburd, 1994): • Deterrence - a carryover effect; offenders perceive that there is an elevated risk of detection and arrest • Discouragement – offenders perceive that the effort exceeds anticipated rewards • Police and others often assume a homogenous group of motivated offenders
Synthesising the results: The need for a review • While noted experiments on the extent of displacement and diffusion following focused policing efforts, a systematic appraisal of all the available evidence does not exist • Literature Reviews: • Barr and Pease, 1990; Eck, 1993; and Hesseling, 1994 • No systematic review of diffusion of benefit (Weisburd et al 2006) • A review of displacement and diffusion effects among situational crime prevention (SCP) (Guerette and Bowers, 2009)
Bowers et al 2011: Spatial Displacement and Diffusion of Benefit among Geographically Focused Policing InitiativesInclusion 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
Inclusion criteria • Quantitative measure of crime • For both the ‘Treatment’ area and a displacement/diffusion ‘catchment’ area • Pre and post (or pre and during) measures necessary • Those without a control area were considered • Not included in the meta-analysis • Intervention was ‘geographically focused’ to a local area • Very large scale not included (e.g. Entire city) • A series of ‘types’ of area • Census blocks, police zones/beats/divisions/precincts, estates, districts, suburbs, block areas, series of roads, neighbourhoods
Inclusion criteria • Any point in time and any location • Study written in English • Both published and unpublished studies included • Article reported original research findings • Not meta-analysis or reviews • Where same project reported in multiple places most detailed study used
Search Strategy • A keyword search of electronic abstract databases • A review of bibliographies of existing displacement reviews (e.g. Barr and Pease, 1990; Eck, 1993; Hesseling, 1994; Guerette and Bowers, 2009) and reviews of the effectiveness of focused policing initiatives (e.g. Braga,2007; Weisburd et al., 2008). • Forward searches for works that have cited key displacement publications • A review of research reports of professional research and policing organizations • Hand search of pertinent journals • A specific Boolean search term
Design Treatment area Control area Treatment Control N(%) catchment catchment RCT X X X X 4(9%) 1(2%) RCT X X X Quasi - Experimental X X X X 2(5%) Quasi - Experimental X X X 12(27%) Quasi - Experimental X X 25(57%) Hierarchy of Evidence
Narrative Review • 55% of the studies reported finding no spatial displacement; compared to 39% who did find evidence. • Diffusion of benefit was found in 43% of studies and not in 5%. • For the remaining 7% (displacement) and 52% (diffusion) of studies respectively, outcomes are unknown mainly due to the fact that they were not explicitly examined by the study authors
Meta-analysis • Pre/post counts of crime for 2/3/4 areas commonly reported • In some cases it is possible to calculate/convert figures to get these counts • Odds Ratio 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 area
Context and Mechanism • Context and mechanism (Pawson and Tilley 1997, Pawson 2006) • descriptive analysis of the extent to which displacement and diffusion is found by the authors across a number of different contexts. • Diffusion was found slightly more often by POP-based initiatives than those representing increased police presence in areas (47% vs 39%); • by those using data from sources other than recorded crime (56% vs 40%); • those with larger physical area coverage (50% for large, 33% for medium and 36% for small); • those conducted in Australia compared to the US and the UK (67%, 43% and 40% respectively)
Theoretical Centrality • In many cases (59%) -but by no means all of them - the research was informed by prior research or theory concerning the possibility that crime might be displaced. • Degree of variation in terms of how centrally the issue of displacement was examined; in 18 (41%) of cases it was centrally examined; in 10 cases there was a brief discussion; it was examined peripherally in 7 cases and undertaken as post-hoc analysis in a further 7 studies.
A final word on Evidence • RCTs • High standard of internal validity: good quality experiments • Often external validity is limited • Do not always consider or document context • Very seldom consider mechanism (the way in which intervention X works in situation Y) • Often take implementation success as a given (like in medical trials) • Meta-analysis • Is only one synthesis approach • Has the advantage of transparency • Tends to favour quantitative evidence and RCTs
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 catchment areas are non-significant but there is a trend in favour of a diffusion of benefit • For successful interventions, there is a diffusion of benefit and the mean effect is statistically significant • For RCTs, there is a diffusion of benefit and the mean effect is statistically significant • Results for subgroups • POP studies are more effective at controlling crime in hot spots than traditional policing techniques • POP studies slightly more likely to diffuse benefit than focused policing alone, interventions that cover large or small areas as likely to diffuse benefits
Using the evidence in practice • Focusing police efforts on hot spots is worthwhile • There is no evidence that it will lead to problems elsewhere • POP seems to be particularly promising from the range of available approaches • Sharing evidence should help determine the right type of intervention for a particular context and any implementation issues to be aware of.