An Investigation into Compound Likelihood Ratios for Forensic DNA Mixtures

Simple propositions are defined as those with one POI and the remaining contributors unknown under H(p) and all unknown contributors under H(a). Conditional propositions are defined as those with one POI, one or more assumed contributors, and the remaining contributors (if any) unknown under H(p), and the assumed contributor(s) and N unknown contributors under H(a). In this study, compound propositions are those with multiple POI and the remaining contributors unknown under H(p) and all unknown contributors under Ha. We study the performance of these three proposition sets on thirty-two samples (two laboratories × four NOCs × four mixtures) consisting of four mixtures, each with N = 2, N = 3, N = 4, and N = 5 contributors using the probabilistic genotyping software, STRmix™. In this study, it was found that conditional propositions have a much higher ability to differentiate true from false donors than simple propositions. Compound propositions can misstate the weight of evidence given the propositions strongly in either direction.