The FBI’s validation of STRmix™ v2.11 according to the SWGDAM guidelines for the validation of probabilistic genotyping systems

STRmix™ v2.11 was internally validated at the FBI laboratory in accordance with the SWGDAM Guidelines for the Validation of Probabilistic Genotyping Systems to enable the upgrade from the previously validated version 2.4. Mixtures were designed containing DNA from 2 to 7 persons, ranging from 8 pg to 2 ng and including both degraded and non-degraded material. Various mixture proportions were evaluated to maximize coverage of the factor space, including mixtures with minor donor proportions < 1%. Based on ground truth and intuitive alignment with expert interpretations, these studies demonstrated that STRmix™ v2.11 is well suited for the interpretation of evidentiary DNA typing results that originate from 1 to 5 donors generated with CE injection times of both 12 & 24 s in a multi-laboratory system. Instances of unintuitive diagnostics usually originated from profiles amplified with large amounts of template DNA (i.e., 2 ng which is greater than our normal target of 1 ng). In addition, these excessive template amplifications may cause large peak height variances, as well as large Gelman-Rubin values indicating potential non-convergence of chains and false exclusions of known contributors. Studies assessing number of contributor (NOC) assignments were conducted allowing for STRmix™ analysis of mixtures according to both the actual NOC (i.e., informed ground truth NOC only accounting for alleles above the AT) as well as the examiner determined NOC. These resulted in limited differences between the LR results obtained.