Taylor et al. have described a method to weigh the probability that the DNA profile under different values of N and incorporate this into a likelihood ratio (LR), and the current paper reports exploration of the performance of this variable number of contributors (varNoC) method programmed within the probabilistic genotyping software STRmix™.
Uncertainty in the assignment of the number of contributors (NoC) can be encountered, particularly in higher-order mixtures, where alleles may be shared between contributors, may have dropped out, or may be masked by the stutter artefacts or allelic peaks of a more dominant contributor. Most probabilistic genotyping software requires the assignment of NoC prior to interpretation. NoC has been described as a nuisance parameter. In the current project, the desired combination of performance and runtime was obtained using the default STRmix™ version 2.7 MCMC settings in conjunction with a 2.5 percent hyper-rectangle range, at least 10,000 naïve MC iterations and 8 MCMC chains. The varNoC LR demonstrated the typical sensitivity and specificity behavior observed in previous studies, with a high level of reproducibility given repeat analyses. Profiles previously demonstrating ambiguity in the NoC assigned using conventional estimation methods were able to be reliably interpreted and a varNoC LR assigned. (publisher abstract modified)