Linking serial homicide - towards an ecologically valid application

Purpose–Crime linkage analysis (CLA) can be applied in the police investigation-phase to sift through adatabase to find behaviorally similar cases to the one under investigation and in the trial-phase to try toprove that the perpetrator of two or more offences is the same, by showing similarity and distinctivenessin the offences. Lately, research has moved toward more naturalistic settings, analyzing data sets that areas similar to actual crime databases as possible. One such step has been to include one-off offences inthe data sets, but this has not yet been done with homicide. The purpose of this paper is to investigatehow linking accuracy of serial homicide is affected as a function of added hard-to-solve one-off offences.Design/methodology/approach–A sample (N = 117–1160) of Italian serial homicides (n = 116) andhard-to-solve one-off homicides (n = 1–1044, simulated from 45 cases) was analyzed using a Bayesianapproach to identify series membership, and a case by case comparison of similarity using Jaccard’scoefficient. Linking accuracy was evaluated using receiver operating characteristics and by examiningthe sensitivity and specificity of the model.Findings–After an initial dip in linking accuracy (as measured by the AUC), the accuracy increased asmore one-offs were added to the data. While adding one-offs made it easier to identify correct series(increased sensitivity), there was an increase in false positives (decreased specificity) in the linkagedecisions. When rank ordering cases according to similarity, linkage accuracy was affected negativelyas a function of added non-serial cases.Practical implications–While using a more natural data set, in terms of adding a significant portion ofnon-serial homicides into the mix, does introduce error into the linkage decision, the authors concludethat taken overall, the findings still support the validity of CLA in practice.Originality/value–This is the first crime linkage study on homicide to investigate how linking accuracy isaffected as a function of non-serial cases being introduced into the data.