Prospective crime mapping in operational context Final report

A simple time-series analysis (see Appendix 3) confirmed that the two areas followed asimilar trend and experienced a similar volume of burglary for the two years before the pilot.At the start of the pilot (August-September) the volume of burglary rose in both areas, afterwhich it remained somewhat stable in the comparison area but fell in the pilot area. For themonths of January and February 2006 the volume of burglary in the pilot area was the lowestit had been for at least the last five years, being less than half the volume for the equivalentperiod of time in the previous year. 7 For reference, also shown in Figure 6.1 are the times atwhich the major implementation outputs of the pilot began.At this point, it is perhaps useful to provide the reader with a little more contextual informationregarding other policing initiatives implemented in ‘A’ Division before or around the time of thepilot. Interviews with the Command Team, LIOs and the analysts for the Division, suggestedthat the only intervention implemented across the Division was the prioritisation of prolific andpriority offenders (PPOs). This began around February 2005 and is ongoing. The aim of theintervention is to target prolific offenders, those who commit the bulk of offences, across theentire BCU with the aim of detecting and consequently reducing crime.Given the focus of this intervention, it is plausible that this could have impacted upon theincidence of burglary before and during the pilot period. To see if any changes in burglaryoffences observed in ‘A’ Division were likely to be attributed to this strategy, the number ofdetections recorded for the seven-month periods before and during the pilot were consideredand compared to those in ‘C’ Division (which also focused on PPOs). This analysis revealedthat in ‘A’ Division the number of detections per 1,000 burglaries increased slightly over time,but less so than it did in ‘C’ Division for the same period of time. Moreover, in the pilot areathe rate of detections was a little lower for both periods than it was for the same period of timeone year earlier, whereas for the comparison area the reverse was true. This pattern ofresults would suggest that changes in the pilot area over time are unlikely to be attributable tothe targeting of prolific offenders.Complicated analyses could be conducted to attempt to determine whether the reduction inthe incidence of burglary observed was statistically significant. Readers interested in whatsuch analyses might show are directed to Appendix 3 of this report. However, as alreadydiscussed it is proposed that the interpretation of such analyses would be unclear asimplementation of the pilot on the ground was so limited. Thus, in this section a simplemeasure is presented as a guide to the changes observed. The metric computed, an oddsratio, merely contrasts the change in the intervention and comparison areas before and afterintervention. An odds ratio of one indicates that the changes in the two areas werecommensurate, suggesting no change in the pilot area. An odds ratio of greater (less) thanone suggests a reduction (increase) in the intervention area relative to the change observedin the comparison area. The statistical significance of the odds ratio can also be computed(see Lipsey and Wilson, 2001) by estimating the standard error of the value derived.This technique, which is readily interpretable, has been frequently used in researchconcerned with what works in reducing crime (for examples, see Welsh and Farrington, 2006;Gill and Spriggs, 2005), but is not without it critics, particularly for analyses conducted at thesmall area level (for which fluctuations over time may occur even in the absence ofintervention: Marchant, 2005). However, the problems articulated about this approach arelikely to be less problematic for analyses conducted at the BCU level, for which the variationover time is less of an issue than for smaller areas (see Farrington and Welsh, 2006). Thus,the approach is used here because it provides a simple assessment of how things changed inthe pilot area relative to the comparator.7 Perhaps ironically, it was at this point in time, during which the burglary rate had remained stable for the last year,that ‘A’ Division was selected as the pilot location.60