Response to: Commentary on: Bright et al. (2018) Internal validation of STRmix™ – A multi laboratory response to PCAST, Forensic Science International: Genetics, 34: 11–24

Commentary on: Thompson WC. Uncertainty in probabilistic genotyping of low template DNA: a case study comparing STRmix™ and TrueAllele™. J Forensic Sci. 2023;68 (3):1049–63. doi: 10.1111/1556‐4029.15225

“…Given this statement by Thompson, with which we agree, we suggest that the central theme of his paper is not supported. For example, the title: "Uncertainty in probabilistic genotyping of low template DNA: A case study comparing STRmix™ There are a number of studies now comparing different software (see for example [1][2][3][4][5]). The Riman et al [2] study is quoted by Thompson but has accepted faults noted by Riman et al more extensively in [3].…”

“…However, to be useful this knowledge needs to be transformed into which, if any, of the software is credible and if so under what conditions. This is best undertaken by calibration [4] (see [5,6] for some STRmix™ calibrations). We do commend Thompson's locus by locus examination and the scoring against his subjective expectations.…”

Proposed Framework for Comparison of Continuous Probabilistic Genotyping Systems amongst Different Laboratories

“…The values for w m will vary from laboratory to laboratory. This is because each laboratory must model epg artefacts and peak height variance for the particular conditions in their laboratory, and these models inform the various w. At first glance, and this is certainly the view of Buckleton et al [36], this suggests that LRs reported by different laboratories cannot be compared. While it is true that not all LRs can be compared, we can define specific conditions for which a subset of LRs can be compared.…”

“…McNevin et al [35] have previously suggested a method for assessing reproducibility and defining credible intervals for LRs derived from the same DNA extracts (not electropherograms) and calculated by STRmix in particular and continuous PG in general. This was met with some scepticism by Buckleton et al [36] who contend, firstly, that there are "multiple reasonable answers in the case of evidence from one extract" [36,37] and, secondly, that it is sufficient to calibrate the LRs generated by PG from multiple laboratories using the method of Ramos and Gonzalez-Rodriguez [38]. In summary, this last method uses the LRs and a prior odds ratio from known numbers of contributors and non-contributors submitted by multiple laboratories to calculate a posterior odds ratio.…”

“…Peak height variance and degradation have been posited by Buckleton et al [36] as another reason LRs cannot be compared amongst laboratories. However, at any particular locus for any particular kit, we are restricting w m such that each laboratory finds w i → 0 and w j are the same for all laboratories.…”