Effect of Private Police on Crime: Evidence from a Geographic Regression Discontinuity Design

Past literature in criminology has shown that increases in policing can reduce crime. Generally, most rigorous studies have been done on temporary deployments of extra police, or “episodic” changes rather than permanent changes. To better understand effects of a sustained increase in police deployment, a group of researchers working in Philadelphia decided to look at the effect of the University of Pennsylvania Police Department (UPPD), which patrols within a specific boundary surrounding the university, on the amount of crime happening in the surrounding area.

The UPPD provide supplemental police services in University City, a district of Philadelphia including the university and surrounding neighborhoods. They are the largest privately funded publicly certified police force in Pennsylvania. Though the Philadelphia Police Department (PPD) serves all of Philadelphia, within the patrol zone boundary of UPPD, those forces are supplemented by university police officers. At any given time, there are between 6-8 PPD officers in the entire University City district, as well as some private security officers, and UPPD has 16 officers within the Penn patrol boundary at all times. As the number of officers within the patrol zone is approximately doubled, the increase in policing coming from the UPPD officers is certainly significant.

The researchers used a geographic regression discontinuity design to study the effects of the extra police officers  by taking advantage of the arbitrary boundary in which the UPPD patrols. Using common indicators that affect crime rates, such as population characteristics and land use, they found that the blocks just inside and just outside the boundary were essentially identical in terms of crime rate prediction. This means that a difference in crime rates just inside and outside the boundary (or what is known as a discontinuity) could have been caused by the treatment applied within the boundary — in this case, the increased police presence.

The study used data from UPPD of all crimes in University City from 2005-2010. The crimes were classified into street crimes, property crimes, violent crimes, and aggregated into total crimes. The unit of analysis used was a city block, as those were easily identifiable by police and individuals, and are natural settings for police interventions and daily routines. Data on parking tickets and traffic accidents at the block level were also used to serve as control variables, since the UPPD does not issue parking tickets, and should not affect traffic accidents within or outside of the patrol zone boundary.

In the results, there is a clear jump at the boundary in the number of crimes, with different statistical significance depending on the types of crimes and certain estimators used. For all crimes by block, there were between 32 and 61 more crimes in blocks just outside the boundary, which was a significant increase of between 45%-86% relative to the average number of crimes within the boundary. To check the results, the researchers tested other scenarios in which blocks were assigned random distances from the boundary and not their actual physical distances. This resulted in only 1.3% of cases with a larger discrepancy between crimes inside and outside the boundary.

For street crimes, which past research has shown to be more easily deterred by police patrols, the increase is a statistically significant 45%-115%. Property crimes show less of an effect, with statistical significance depending on the estimators used, and effects of 27%-60%. Violent crimes also were impacted, with an increase of 119%-153% outside the boundary, though the statistical significance was somewhat dependent on the specification of the estimator.

To check that the likelihood of risky behavior and rule-breaking is not dependent on the geographic location, whether inside or outside the boundary, the researchers used the data parking ticket and traffic accident data to compare the blocks. There was no systematic effect of the patrol zone on the number of these incidents, indicating no substantial differences in those behaviors, at least, on either side of the boundary. This supports the conclusion that increased police presence within a given area reduces crime in that area.

Using the increases found, the study created elasticity estimates for each type of crime. These estimates calculated the amount of change in crime based on the concentration of police per area. The estimates found were very similar to estimates created by previous studies that studied increases in police presence. Though they did not match all previous studies on all types of crime, property crime and violent crime were found to match at least several estimates found in other studies.

Some of the limitations of this study are that the study did not identify the specific mechanisms by which police were affecting crime at the boundary of the patrol zone. If the police presence was noticed by criminals, and the boundary was identified, they may have chosen to commit crimes just over the boundary that otherwise they would have committed inside, leading to a greater measured effect since some crime would be displaced from just within to just outside the boundary. However, this noticing of police presence could also have caused criminals to be deterred from committing crimes even just over the boundary, leading to a smaller measured effect, since both blocks just within and just without would have experienced a decrease in crime because of the increased police. An experiment randomly changing the dosage of police officers in an area could help determine whether it was the police themselves, or simply perceived police presence that changed crime levels. Another limitation also may be that there are invisible differences between blocks just inside and just outside the boundary that may or may not be caused by the increased police. The study also did not determine whether reporting behavior changes inside and outside the boundary, or to UPPD versus to PPD (since UPPD would likely have only crimes within the boundary reported to them) which could have led to discrepancies between measured and actual behavior. Further research on these limitations should be done to determine the effects of these on the conclusions made by this study.

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Takeaway:

  • Extra police decrease crime in adjacent city blocks by 43-73%
  • Permanent extra police presence, such as University Police, can reduce crime outside of their “campus” boundaries
  • The results are consistent with other research on increased police presence, specifically for reductions in property and violent crimes

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Reference:

Macdonald, J., Klick, J., & Grunwald, B. (2016). The effect of private police on crime: evidence from a geographic regression discontinuity design. Journal of the Royal Statistical Society: Series A (Statistics in Society), 179(3), 831–846. https://doi.org/10.1111/rssa.12142

 

 

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