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 x four NOCs x four mixtures) consisting of four mixtures, each with N = 2, N = 3, N = 4, and N = 5 contributors using the probabilistic genotyping software, STRmix (TM). 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.