Bayesian Networks and the Value of the Evidence for the Forensic Two-Trace Transfer Problem


  • This work was supported by the Swiss National Science Foundation Grant Number 100014-122601/1.

Additional information and reprint requests:
Simone Gittelson, M.Sc.
Institut de Police Scientifique
Ecole des Sciences Criminelles
Batochime, Quartier Sorge
University of Lausanne
1015 Lausanne


Abstract:  Forensic scientists face increasingly complex inference problems for evaluating likelihood ratios (LRs) for an appropriate pair of propositions. Up to now, scientists and statisticians have derived LR formulae using an algebraic approach. However, this approach reaches its limits when addressing cases with an increasing number of variables and dependence relationships between these variables. In this study, we suggest using a graphical approach, based on the construction of Bayesian networks (BNs). We first construct a BN that captures the problem, and then deduce the expression for calculating the LR from this model to compare it with existing LR formulae. We illustrate this idea by applying it to the evaluation of an activity level LR in the context of the two-trace transfer problem. Our approach allows us to relax assumptions made in previous LR developments, produce a new LR formula for the two-trace transfer problem and generalize this scenario to n traces.