Effect on the likelihood ratio when under- and over-assigning the number of contributors to higher order mixtures

The availability of highly sensitive multiplex kits has resulted in the more common observation of low-level and more complex (multiple contributors) DNA profiles within forensic casework. With the emergence of probabilistic genotyping software these complex DNA mixture profiles can now be interpreted. This has led to a shift in questioning within courtroom settings, where the number of contributors (NoC) assigned by the forensic DNA analyst is now often scrutinized. This study aims to assess the likelihood ratio (LR) behavior for the under- and over-assignment of a range of complex mixtures, including six-person mixtures, using the probabilistic genotyping software STRmix™. The mixtures were analyzed using FaSTR™ DNA and a NoC was "blindly" assigned by an analyst (i.e., without prior knowledge of the experimental design). LRs obtained for true contributors and non-contributors, under different NoC assignments, were compared and showed that overestimating NoC may lead to weak adventitious matches, while underestimating can lead to the false exclusions of true contributors. Little effect was observed on the LR assigned to the major contributors to these complex mixtures. Moreover, as the profile complexity increased (higher order mixtures), the impact on the LR for true contributors, when underestimating NoC by one, diminished, with only 35 of the 72 comparisons (~49%) resulting in a smaller LR for the under-assignment. It is paramount for DNA analysts to understand the LR behavior especially when questioned on the witness stand regarding their NoC assignment, as there is no ability to reassess or reinterpret the evidence in real-time.