Abstract

Research Question How accurately can all recorded locations of 97 knife homicides in one year be forecast across all 4,835 Lower Layer Super Output Areas (LSOAs) of London, based only upon all 3,506 known locations of non-fatal knife-injury assaults in the preceding year?

Data All recorded “knife crimes” in the Metropolitan Police Service (MPS) area of London in financial year 2016/17 (Year 1) were manually reviewed to identify all 3506 reported locations of knife-enabled (KE) but non-fatal injuries, as distinct from other events digitally coded as “knife crimes,” such as displaying, threatening with, or carrying knives. All KE homicides in 2017–/18 (Year 2) were then added to the data base.

Methods Each KE injury-assault in Year 1 was classified for occurrence in one of London’s 4,835 LSOAs. The total N of such crimes within each LSOA was summed across all records to divide all LSOAs into seven categories of frequency of KE injury-assaults in 2016–17: zero, 1, 2, 3, 4, 5 and 6-or-more. We then divided the N of LSOAs in each category in 2016–17 (Year 1) into the total N of knife homicides in 2017–18 (Year 2). False positives, false negatives and percentage of homicides targeted were calculated for six different targeting plans.

Findings Over two thirds (69%) of KE homicides in 2017–18 (Year 2) occurred in just 67 (1.4%) of all 4,835 LSOAs, comprising 3.3% of the 2048 LSOAs that had had one or more of the 3,506 KE injury-assaults mapped in 2016–17 (Year 1). The prevalence of one or more KE homicides in Year 2 was higher where there were higher numbers of KE injury-assaults per LSOA in Year 1. Among LSOAs with zero assaults in Year 1, only 1% had a KE homicide in Year 2. Among LSOAs with 6 or more KE injury-assaults in Year 1, 15% had a KE homicide in Year 2. While the risk of homicide was 1,400% higher in the hottest spots of knife assaults than in the coolest, the absolute number of Year 2 KE homicides in those hottest areas was only 6% of the total.

All LSOAs with one or more Year 1 KE assaults had three times as much KE homicide risk than LSOAs with no Year 1 KE assaults. Targeting all 2,048 areas that had one or more Year 1 KE assaults would have yielded a true positive rate of over 3%, and covered 69% of actual homicide locations. The true negative rate for 2,781 areas that would not have been targeted based on having no Year 1 KE injuries would have been 99%.

Conclusions Predicting which local areas are most likely to suffer knife-enabled homicides, based only on recent non-fatal knife injuries, can pinpoint risks of homicide in local areas that are up to 1,400% higher than in most local areas, offering a range of strategies for resource allocation.