Testing the impact of group offending on behavioural similarity in serial robbery

Behavioural case linkage assumes that offenders behave in a similar way across their crimes. However, group offending could impact on behavioural similarity. This study uses robbery data from two police forces to test this by comparing the behavioural similarity of pairs of lone offences (LL), pairs of group offences (GG) and pairs of offences where one crime was committed alone and the other in a group (GL). Behavioural similarity was measured using Jaccard's coefficients. Kruskal–Wallis tests were used to examine differences between the three categories within the linked samples. No statistically significant differences were found for linked GG compared to linked LL pairs. However, differences emerged between GL and the other categories for some behaviours (especially control) suggesting caution should be applied when linking group and lone offences committed by the same perpetrator. Differences between linked and unlinked pairs were assessed using receiver operating characteristic. The results suggest it is possible to distinguish between linked and unlinked pairs based on behaviour especially within the GG and LL categories. There were, however, fewer significant findings for the GL sample, suggesting there may be issues linking crimes where the offender commits one crime as part of a group and the other alone.