Analyzing Private Security Patrols and Crime

Saturday, February 8, 2014

The use of private security patrols is increasing where traditional law enforcement is insufficient to ensure public safety.

On September 23, 2013, with the support of the neighbors on my block and the adjoining block, I launched a crowd-funding campaign for a private security patrol in Lower Rockridge, in an area north of Highway 24, west of College Avenue, east of Telegraph Avenue, and south of Woolsey Street.

Two other campaigns subsequently formed in immediately adjacent neighborhoods ("Lower Rockridge South" and "Lower Rockridge North/East").

The data show that robberies and burglaries in Lower Rockridge doubled from July to September 2013, against a generally flat to declining trend seen in other similar neighborhoods. Trends in October were similar.

Study Results at Three Months
The results in the three months since the patrols started suggest that crime in Lower Rockridge is down by a statistically significant 46.4 percent relative to what it would have been absent the patrols, 44.8 percent after accounting for possible displacement of crime to other areas.

I focused my analysis on robberies and burglaries, as these are the crimes of greater concern to residents in the Lower Rockridge area.

The data show that Lower Rockridge has seen a 33.3 percent decrease in burglaries and robberies in the three months since the patrol started compared to the four weeks prior, while the control group has seen an increase of 24.3 percent. Comparing Lower Rockridge relative to the control group suggests that burglaries/robberies are down 46.4 percent from what they would have been absent the patrols.

A similar result is found when comparing the most recent four weeks to the four weeks prior to the start of the patrol. Lower Rockridge burglaries/robberies were down 60 percent while crime in the control group was down by 10.8 percent. This suggests that burglaries and robberies in the most recent four week period were down 55.2 percent from what they would have been absent the patrols.

Some community members have raised the prospect that crime may have been displaced to other areas. To estimate the effect of displacement on the difference-in-difference calculation (i.e., the change in the treatment relative to the change in the control), recall that to the extent that criminals moved to other areas, the area easily accessible by car in this region is broad. An area including University Avenue in Berkeley south to Fruitvale Avenue in Oakland is all within a 10-minute drive of Lower Rockridge. In the four-week baseline period of October 7 to November 3, CrimeMapping.com shows a total of 424 burglaries and robberies in this broader area

CrimeMapping.com does not include data from Piedmont, so the actual total would be slightly higher. This area includes the 25 burglaries and robberies observed in Lower Rockridge. With a 46.4 percent reduction in crime, I estimate there would have been 13.4 burglaries and robberies had the patrols been in place during the baseline period. The difference is 11.6 burglaries and robberies. If these burglaries and robberies were all displaced instead of deterred and spread evenly across the broader area, crime in the broader area less Lower Rockridge would have increased by less than 3 percent. Adjusting the control area for this maximum displacement gives similar results as before, with an overall three month reduction of 44.8 percent and a most recent four-week reduction of 53.8 percent.

View the full report with its interpretive charts and graphs at http://goo.gl/P3aRXb.