Forensic identification: the island problem and its generalisations
Article first published online: 8 APR 2011
© 2011 The Authors. Statistica Neerlandica © 2011 VVS
Volume 65, Issue 2, pages 202–237, May 2011
How to Cite
Slooten, K. and Meester, R. (2011), Forensic identification: the island problem and its generalisations. Statistica Neerlandica, 65: 202–237. doi: 10.1111/j.1467-9574.2011.00484.x
- Issue published online: 8 APR 2011
- Article first published online: 8 APR 2011
- Received: May 2010. Revised: February 2011.
- Island problem;
- forensic identification;
- weight of evidence;
- posterior odds;
- Bayes’ rule
In forensics it is a classical problem to determine, when a suspect S shares a property Γ with a criminal C, the probability that S = C. In this article we give a detailed account of this problem in various degrees of generality. We start with the classical case where the probability of having Γ, as well as the a priori probability of being the criminal, is the same for all individuals. We then generalize the solution to deal with heterogeneous populations, biased search procedures for the suspect, Γ-correlations, uncertainty about the subpopulation of the criminal and the suspect, and uncertainty about theΓ-frequencies. We also consider the effect of the way the search for S is conducted, in particular when this is done by a database search. A returning theme is that we show that conditioning is of importance when one wants to quantify the ‘weight’ of the evidence by a likelihood ratio. Apart from these mathematical issues, we also discuss the practical problems in applying these issues to the legal process. The posterior probabilities of C = S are typically the same for all reasonable choices of the hypotheses, but this is not the whole story. The legal process might force one to dismiss certain hypotheses, for instance when the relevant likelihood ratio depends on prior probabilities. We discuss this and related issues as well. As such, the article is relevant both from a theoretical and from an applied point of view.