This article discusses issues in and offers recommendations for comparing multiple persons of interest (POI) to DNA mixtures.
In casework, laboratories may be asked to compare DNA mixtures to multiple persons of interest (POI). Guidelines on forensic DNA mixture interpretation recommend that analysts consider several pairs of propositions; however, it is unclear if several likelihood ratios (LRs) per person should be reported or not. The propositions communicated to the court should not depend on the value of the LR. As such, we suggest that the propositions should be functionally exhaustive. This implies that all propositions with a non-zero prior probability need to be considered, at least initially. Those that have a significant posterior probability need to be used in the final evaluation. Using standard probability theory the study combined various propositions so that collectively they are exhaustive. This involves a prior probability that the sub-proposition is true, given that the primary proposition is true. Imagine a case in which there are two possible donors: i and j. This analysis first focused on donor i so that the primary proposition is that i is one of the sources of the DNA. In this example, given that i is a donor, the study further considered that j is either a donor or not. In practice, the prior weights for these sub-propositions may be difficult to assign; however, the LR is often linearly related to these priors and its behavior is predictable. The authors also believe that these priors are unavoidable and are hidden in alternative methods. They term the likelihood ratio formed from these context-exhaustive propositions LR i/i… LR i/i.. was trialed in a set of two- and three-person mixtures. For two-person mixtures, LR i/i was often well approximated by LRij/ja, where the subscript ij describes the proposition that i and j are the donors and ja describes the proposition that j and an alternate, unknown individual (a), who is unrelated to both i and j, are the donors. For three-person mixtures, is often well approximated by LRijk/jka where the subscript ijk describes the proposition that i, j, and k are the donors and jka describes the proposition that j, k, and an unknown, unrelated (to i, j, and k) individual (a) are the donors. In the authors’ simulations, LRij/ja had fewer inclusionary LRs for non-contributors than the unconditioned LR (LRia/aa). (Publisher Abstract)