UCL DEPARTMENT OF SECURITY AND CRIME SCIENCE How can Evidence Reviews make a difference to practice?...

UCL DEPARTMENT OF SECURITY AND CRIME SCIENCE

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.


Braga et al 1999


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


Synthesising the results: Braga et al 2012


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


Moderators: Braga et al 2012


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


Braga et al 1999


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 Initiatives

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


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


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Allatt1984 Situational Crime Prevention

UK Residential Medium No 3 RC

Braga&Bond2008 Problem-oriented Policing

US Multiple Small Yes 4 CFS x x

BragaEtAl1999 Problem-oriented Policing

US Multiple Medium Yes 4 RC/CFS x

Cummings2006 Victim-oriented Policing

AUS Residential Large No 3 RC

Esbensen1987 Foot Patrol US Retail Small No 4 RC x

FarrellEtAl1998 Police Crackdown;

Offender focused

UK Residential Small No 3 RC x

Grogger2002 Hot spots; Civil Injunctions

US Multiple Medium No 3 RC x

Higgins&Coldren2000 Problem-Oriented

Policing; Civil Abatement

US Residential Large No 3 RC

MazerolleEtAl2000 Civil Abatement US Multiple Large Yes 4 CFS

McGarrellEtAl2001 Focused Patrol US Residential Medium No 3 RC

Press1971 Police Saturation US Residential Large No 3 RC

Segrave&Collins2005 Community-oriented Policing

AUS Residential & Retail

Large No 3 RC x

Sherman&Rogan1995 Hot Spots Policing US Residential Large No 3 RC x

Weisburd&Green1995 Police Crackdown

US Residential & Retail

Medium Yes 4 CFS x

In the meta-analysis


Hierarchy of Evidence

Design Treatment area

Control area Treatmentcatchment

Control catchment

N(%)

RCT X X X X 4(9%)RCT X X X 1(2%)

Quasi - Experimental X X X X 2(5%)Quasi - Experimental X X X 12(27%)Quasi - Experimental X X 25(57%)


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


Best case scenario (N=16)

Odds Ratio

Favors TreatmentFavors ControlWeighted Mean OR (RDM ef f ects)

Allatt1984 (Pre-Post, Catch 2)

BragaEtAl1999 (Crime)

Braga&Bond2008

Cummings2006 (Treatment 1)

Esbensen1987 (Disorder)

FarrellEtAl1998 (Pre-Post)

Grogger2002 (Catchment ctrl)

Higgins&Coldren2000

MazerollePriceEtAl2000

McGarrellEtAl2001

Press1971(Auto Thef t, Outside)

Ratclif f e et al. 2010

Segrav e&Collins2005 (Dis, Catch 1)

Sherman&Rogan1995 (Catch 2)

Wagers2007

Weisburd&Green1995

-2 -1 0 1 2 3 4 5

Treatment

Catchment

-2 -1 0 1 2 3 4

0.0

0.5

1.0

1.5

2.0

2.5

3.0

Treatment Odds Ratio

Ca

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Reduction in both areas

Reduction in Treatment

Increase in Catchment

Increase in Treatment

Decrease in Catchment

Increase in both areas


Worst case scenario (N=15*)

Odds Ratio

Favors TreatmentFavors ControlWeighted Mean OR (RDM ef f ects)

Allatt1984 (Pre-Dur, Catch 1)

BragaEtAl 1999 (CFS)

Braga&Bond2008

Cummings2006 (Treatment 2)

Esbensen1987 (Index Crimes)

FarrellEtAl1998 (Pre-During)

Grogger2002 (No catchment ctrl)

Higgins&Coldren2000

MazerollePriceEtAl2000

McGarrellEtAl2001

Press1971(Burglary , Outside)

Ratclif f e et al. 2010

Segrav e&Collins2005(Prop, Catch 3)

Sherman&Rogan1995 (Catch 1)

Wagers2007

-2 -1 0 1 2 3 4 5

Treatment

Catchment

-2 -1 0 1 2 3 4

0.0

0.5

1.0

1.5

2.0

2.5

3.0

Treatment Odds Ratio

Ca

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Reduction in both areas

Reduction in Treatment

Increase in Catchment

Increase in Treatment

Decrease in Catchment

Increase in both areas


All effect sizes (N=52)


Pop studies

0.6 0.8 1.0 1.2 1.4

05

10

15

Treatment Mean OR (RDM effects)

All

18

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Favors TreatmentFavors Control

0.6 0.8 1.0 1.2 1.4

05

10

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Catchment Mean OR (RDM effects)

All

18

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Focused policing alone

0.6 0.8 1.0 1.2 1.4

05

01

00

15

02

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Treatment Mean OR (RDM effects)

All

48

0 P

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0.6 0.8 1.0 1.2 1.4

05

01

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15

02

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Catchment Mean OR (RDM effects)

All

48

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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.


Braga et al 2012 also measured displacement…


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.

UCL DEPARTMENT OF SECURITY AND CRIME SCIENCE How can Evidence Reviews make a difference to practice?...

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