This article reports on a method that uses the distribution of likelihood ratios obtained from nondonors.
Since the reporting of a likelihood ratio (LR) calculated from probabilistic genotyping software has become more popular since 2015 and has enabled the use of more complex mixtures at court. The meaning of “inconclusive” LRs and how to communicate the significance of low LRs at court is now important. The reporting of a likelihood ratio (LR) calculated from probabilistic genotyping software has become more popular since 2015 and has enabled the use of more complex mixtures at court. The meaning of “inconclusive” LRs and how to communicate the significance of low LRs at court is now important. The nondonor distribution is useful for examining calibration and discrimination for profiles that have produced LRs less than about 104. In the current project, a range of mixed DNA profiles of varying quantity were constructed and the LR distribution considering the minor contributor for a number of nondonors was compared to the expectation given a calibrated system. It was demonstrated that conditioning genotypes should be used where reasonable, given the background information to decrease the rate of nondonor LRs above 1. In all 17 cases examined, the LR for the minor donor was higher than the nondonor LRs, and in 12 of the 17 cases, the 99.9 percentile of the nondonor distribution was lower when appropriate conditioning information was used. The output of the tool is a graph that can show the position of the LR for the person of interest set against the nondonor LR distribution. This may assist communication between scientists and the court. (publisher abstract modified)