Building up on the introduction of the Cambridge Crime Harm Index (CHI) – a new framework designed to quantify crime based on damage caused to its victims and society – top criminologists at the University of Cambridge have put the index into action, teaming up the Cambridge Constabulary for an experiment in Peterborough. Based on the calculations specified in the CHI the researchers found that an extra 21 minutes of proactive police patrol in crime ‘hot spots’ could prevent 86 assaults a year (or incidents of the equivalent crime ‘harm value), saving potential prison costs to the public of eight years of imprisonment. “Any other investment in policing can now be challenged to match the benefits of foot patrols in preventing the equivalent of either 86 assaults, or six burglaries, or six sexual crimes,” explains co-author on the study, Professor Lawrence Sherman, Director of the Cambridge Institute of Criminology

Working alongside the police force, Professor Sherman and his team identified 72 crime hot spots in Peterborough (defined as small urban areas, streets or intersections, where there is a concentration of crime). 34 of those locations were prescribed an extra 21 minutes of police foot patrol per day, while the remaining 38 were designated as the control locations, over the course of the year-long experiment. “The researchers found that, on average per hot spot, 39% fewer crime incidents were reported by victims and 20% fewer 999 emergency calls to the police occurred in the 34 treated hot spots compared with the 38 control hot spots,” reads the University of Cambridge feature corresponding to the study published in The Journal of Experimental Criminology last month.

The extra 21 minutes of patrolling per day amounted to 3,094 man hours across all treated hot spots, which roughly equates to two fulltime salaries for Police Community Support Officers (PCSOs): GBP £50,000 (USD $67,000) annually. Using the CHI, the researchers calculated that crimes prevented by increased PCSO presence in the randomly selected 34 hot spots amounted to 2,914 days – around eight years – of prison time, at a potential cost to the taxpayer of GBP £280,000 (USD $375,000), based on English sentencing guidelines. That’s more than a five-to-one return on investment. “The use of the Cambridge Crime Harm Index and the Peterborough cost-effectiveness results provides a like-for-like metric to challenge those who demand more PC or PCSO time in patrolling schools, low-crime neighbourhoods, or traffic accident hot spots,” Sherman explains. “This study should give both Police and Crime Commissioners and Chief Constables a benchmark for evaluating any other uses of police time other than hot spots patrols.”

PCSOs are civilian members of police staff in the UK, and due to budgetary constraints, they are the only department that currently conduct proactive and visible foot patrols, whereas Police Officers (POs) tend to patrol in vehicles and tend only to respond to emergencies. In the experiment, participating PCSOs were instructed to focus on being visible. The results showed that every extra PCSO visit to treatment hot spots decreased calls for service by approximately 34, with the number of crimes declining by around four. “The experiment suggests that the number of visits to each hot spot may matter more than the total minutes – as if each time the police arrive they renew their deterrent effect on crime,” explains Dr Barak Ariel of the Lee Centre of Experimental Criminology and lead researcher on the Peterborough experiment.

“These findings suggest that the probability of encountering an officer is more important than the powers that officer has, and that the frequency and duration of proactive patrolling deserves far more attention,” adds Sherman.