Abstract
Objectives
Prior meta analyses of hot spots policing show that the approach reduces crime, but report relatively small mean effect sizes based on Cohen’s d. The natural logarithm of the relative incidence rate ratio (log RIRR) has been suggested as a more suitable effect size metric for place-based studies that report crime outcomes as count data. We calculate the log RIRR for hot spots policing studies to assess whether it changes interpretation of hot spots policing’s impact on crime.
Methods
Cohen’s d and log RIRR effect size metrics were calculated for 53 studies representing 60 tests of hot spots policing programs. Meta-analytic techniques were used to compare the estimated impacts of hot spots policing on crime and investigate the influence of moderating variables using the two differing effect size metrics.
Results
The Cohen’s d meta-analysis revealed a “small” statistically significant mean effect size favoring hot spots policing in reducing crime outcomes at treatment places relative to control places (d = .12) of approximately 8.1%. In contrast, the log RIRR meta-analysis suggests that hot spots policing generated a more substantive 16% (d = .24) statistically significant crime reduction. The two metrics also produced differing rank orders in magnitudes of effect for the same studies.
Conclusion
Cohen’s d provides misleading results when used to calculate mean effect size in place based studies both in terms of the relative ranking of the magnitude of study outcomes, and in the interpretation of average impacts of interventions. Our analyses suggest a much more meaningful impact of hot spots policing on crime than previous reviews.
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Notes
While some meta-analytic scholars had raised concerns about using Cohen’s d to represent program effects in studies that reported area-based crime count data (Farrington et al. 2007; Farrington and Welsh 2013), the general approach used in the ongoing hot spots policing systematic review was considered an acceptable methodology. Indeed, the effect size approach used in the hot spots policing review passed ongoing scientific scrutiny by Campbell Collaboration methods reviewers and high-quality social science journal peer reviewers.
In their meta-analysis of an English national quasi-experimental multi-site evaluation of the effects of closed-circuit television (CCTV), Farrington et al. (2007) derived this adjustment for overdispersion through a linear regression analysis of 70 sets of monthly crime counts in treatment, control, buffer, and police division areas. As they described (Farrington et al. 2007, pp. 36–37), “For each area in each year, the total number of crimes N was compared with V/N, where V is the estimated variance of the number of crimes (based on monthly numbers). In a Poisson process, V/N = 1. It was clear that V/N increased with the total number of crimes. The correlation between V/N and N was 0.77 (p < 0.0001). A linear regression analysis showed that V/N = 0.0008 * N + 1.2.”.
Wilson (2020) suggests the following formula to estimate the quasi-Poisson overdispersion parameter: \(\emptyset = \frac{1}{{\sum n_{k} - 2}} \sum \frac{{s_{k}^{2} \left( {n_{k} - 1} \right)}}{{\overline{{x_{k} }} }},\) where \(\overline{{x_{k} }}\) is the mean count (or rate) for the treatment and control areas both pretest and post-test, resulting in four means, sk is the standard deviation for each of the four mean counts, and nk is the number of counts contributing to each mean and standard deviation. For studies that reported the log IRR and its standard error, we used those metrics in the meta-analysis as these count regression models already adjusted for overdispersion.
When included studies reported an aggregate crime category (such as total incidents or total calls for service), we calculated our effect size measures based on the provided metrics. In the absence of an aggregate crime category, we combined all reported outcomes into an overall average effect size statistic (designated “combined” in the meta-analysis forest plots).
Q = 67.516, degrees of freedom = 27, p < 0.001, I2 = 60.009.
Q = 78/944, degrees of freedom = 59, p < 0.05, I2 = 25.264.
For the Cohen’s d meta-analysis, the between group Q = 20.573, degrees of freedom = 1, p < 0.001. For the log RIRR meta-analysis, the between group Q = 61.658, degrees of freedom = 1, p < 0.001.
https://www.rand.org/well-being/justice-policy/centers/quality-policing/cost-of-crime.html (accessed May 22, 2020).
While the identification process yielded 56 distinct violent crime hot spots, the Jersey City Police Department’s Violent Crimes Unit only had the resources to implement the problem-oriented policing intervention at 12 treatment locations that were matched in pairs to 12 control locations (one member of each pair was allocated to treatment and control conditions). As such, the resulting randomized controlled trial only considered N = 24 total violent crime hot spots. For discussion of the hot spots identification process and the implementation of the randomized controlled trial, please see Braga (1997) and Braga et al. (1999). We used FBI UCR citywide counts to estimate the number of aggravated assaults because the randomized experiment included simple assaults and aggravated assaults in one total assault incident outcome.
https://www.ucrdatatool.gov/Search/Crime/Local/RunCrimeJurisbyJurisLarge.cfm (accessed May 22, 2020).
https://www2.fbi.gov/ucr/cius_04/documents/CIUS_2004_Section2.pdf (accessed May 22, 2020).
https://www.rand.org/well-being/justice-policy/centers/quality-policing/cost-of-crime.html (accessed May 17, 2020).
When we limited our main effects meta-analysis to the hot spots policing studies included in the Blueprints review (Buckley et al. 2020) that included the appropriate information to estimate log RIRR effect size metrics (N = 18), our meta-analysis estimated a 10.3% reduction in crime at treatment places relative to control places. Using the violent crime counts and cost estimates presented in the main text, the much more restrictive Blueprints review suggests that the hot spots policing intervention would have generated roughly $7.8 million in cost savings if the program was applied to all violent crime hot spots in Jersey City for one year and $2.3 million if applied to violence in all crime and disorder hot spots in Lowell for one year.
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Braga, A.A., Weisburd, D.L. Does Hot Spots Policing Have Meaningful Impacts on Crime? Findings from An Alternative Approach to Estimating Effect Sizes from Place-Based Program Evaluations. J Quant Criminol 38, 1–22 (2022). https://doi.org/10.1007/s10940-020-09481-7
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DOI: https://doi.org/10.1007/s10940-020-09481-7