The impact of data degradation and sample size on the performance of two similarity coefficients used in behavioural linkage analysis.

In order to determine whether a series of unsolved crimes has been committed by the same offender, the police often must rely on an analysis of behavioural evidence. When carrying out this task, some type of similarity coefficient is typically relied on to assess the degree of behavioural stability and distinctiveness that exists across a set of crimes and questions inevitably arise as to which coefficient to use. In cases of juvenile sex offences, research has suggested that a taxonomic similarity index outperforms the most commonly used metric at the moment, Jaccard's coefficient, especially under conditions of data degradation (missing data). However, recent research has failed to replicate this result in cases of serial homicide and burglary, especially when relatively large sample sizes are used. The current study provides further support for these recent findings using adult serial sexual assault data. Across a range of conditions, the current study demonstrates that Jaccard's coefficient slightly outperforms the taxonomic similarity index on a measure of linking accuracy. Potential explanations for the results are provided, implications are discussed, and future research directions are presented.