Behavioural Case Linkage in Personal Robbery

Case linkage uses crime scene behaviours to identify series of crimes committed by the same offender. The research presented here tests the underlying assumptions of case linkage (behavioural consistency and behavioural distinctiveness) by comparing the behavioural similarity of linked pairs of offences (i.e. two offences committed by the same offender) and unlinked pairs of offences (i.e. two offences committed by different offenders). It was hypothesised that linked pairs would be more behaviourally similar than unlinked pairs thereby providing evidence for these two assumptions. Logistic regression and receiver operating characteristic analyses were used to explore which behaviours can be used to reliably link personal robbery offences using samples provided by two police forces (one urban and one rural). The method of generating unlinked pairs was then refined to reflect how the police work at a local level, and the success of predictive factors re-tested. This research provided evidence supporting the assumptions with linked pairs displaying more similarity than unlinked pairs across a range of behavioural domains. Inter- Crime Distance and Target Selection emerged as the most useful linkage factors with promising results also found for Temporal Proximity and Control. No evidence was found to indicate that either the Approach used or the Property stolen were useful for linkage. The addition of extra behaviours into domains improved performance in some instances but not substantially. The potential impact of group offending on the assumptions was also tested. Although there were some differences found between group and lone robberies, the research demonstrated that case linkage remains feasible provided that the offences under examination are either group or lone in nature rather than a mixture of the two. A supplementary study gathering the views and experiences of police crime analysts regarding case linkage helped put these new quantitative findings into operational context.