Examining the Role of Similarity Coefficients and the Value of Behavioural Themes in Attempts to Link Serial Arson Offences HOLLY ELLINGWOOD 1, *, REBECCA MUGFORD 1 , CRAIG BENNELL 1 , TAMARA MELNYK 1 and KATARINA FRITZON 2 1 Department of Psychology, Carleton University, Canada 2 Faculty of Humanities and Social Sciences, Bond University, Australia Abstract When relying on crime scene behaviours to link serial crimes, linking accuracy may be influenced by the measure used to assess across-crime similarity and the types of behaviours included in the analysis. To examine these issues, the present study compared the level of linking accuracy achieved by using the simple matching index (S) to that of the commonly used Jaccard’s coefficient (J) across themes of arson behaviour. The data consisted of 42 crime scene behaviours, separated into three behavioural themes, which were exhibited by 37 offendersacross 114 solved arsons. The results of logistic regression and receiver op- erating characteristic analysis indicate that, with the exception of one theme where S was more effective than J at discriminating between linked and unlinked crimes, no significant differences emerged between the two similarity measures. In addition, our results suggest that thematically unrelated behaviours can be used to link crimes with the same degree of accuracy as thematic- ally related behaviours, potentially calling into the question the importance of theme-based approaches to behavioural linkage analysis. Copyright © 2012 John Wiley & Sons, Ltd. Key words: linkage analysis; simple matching index; Jaccard’s coefficient; ROC analysis; logistic regression; serial arson One of the challenges police investigators sometimes face is the task of correctly linking unsolved crimes to the same offender (Grubin, Kelly, & Brunsdon, 2001). This is a particularly difficult task when physical evidence is not available for analysis. Under these circumstances, investigators often rely on behavioural information obtained from crime scenes to establish any crime linkages by using a technique known as behavioural linkage analysis (BLA) (Woodhams, Hollin, & Bull, 2007). The need to rely on BLA may be particularly common in cases of serial arsons because physical evidence will often be destroyed by the fire. For it to be possible to successfully link serial arsons by using crime scene behaviours, one must consider two key assumptions (Canter, 1995). First, arsonists must exhibit *Correspondence to: Holly Ellingwood, Department of Psychology, Carleton University, 1125 Colonel By Drive, Ottawa, Ontario, K1S 5B6, Canada. E-mail: hellingw@connect.carleton.ca. Journal of Investigative Psychology and Offender Profiling J. Investig. Psych. Offender Profil. 10:1–27 (2013) Published online 19 June 2012 in Wiley Online Library (wileyonlinelibrary.com). DOI: 10.1002/jip.1364 Copyright © 2012 John Wiley & Sons, Ltd.