Likelihood Ratio Calculation Using LRmix Studio

Abstract

LRmix Studio performs statistical analyses on forensic casework samples by calculating a likelihood ratio (LR) following a semi-continuous, unrestricted approach. The software utilizes a basic probabilistic model allowing the comparison of two alternative hypotheses regarding the evidence profile to include known and/or unknown contributors, for a maximum of a 4-person mixture. Other statistical factors that are included in this model are the incorporation of multiple probability of drop-out values, probability of drop-in, a correction factor for population substructure, assumed contributor inclusion, and inclusion of an unknown relative in the defense hypothesis. A range of plausible probability of drop-out values can be calculated for various contributors and hypotheses based on a Monte Carlo probability method and included in the likelihood ratio calculation. The software also includes several ways to test the validity and robustness of the probabilistic model. A sensitivity analysis can be performed by calculating likelihood ratios for the given profile against a range of drop-out values. Additionally, a non-contributor test can be performed on the crime scene sample and the chosen LR parameters to test the robustness of the model. This can give a point of comparison of the likelihood ratio generated for the person of interest (POI) compared to "random man" profiles generated from uploaded allelic frequencies. Finally, the analysis can be printed in a well-structured and user-friendly report that includes all analysis parameters. Within this chapter, the reader will learn the steps to calculate a likelihood ratio using the semi-continuous software, LRmix Studio. Additional tools supplied through the software will also be explained and demonstrated.

Keywords: Forensic genetics; Forensic statistical analysis; LRmix Studio; Likelihood ratio; Mixture interpretation; Probabilistic modeling; Probability of drop-out.