Abstract
Continuous probabilistic genotyping (PG) systems are becoming the default method for calculating likelihood ratios (LRs) for competing propositions about DNA mixtures. Calculation of the LR relies on numerical methods and simultaneous probabilistic simulations of multiple variables rather than on analytical solutions alone. Some also require modelling of individual laboratory processes that give rise to electropherogram artefacts and peak height variance. For these reasons, it has been argued that any LR produced by continuous PG is unique and cannot be compared with another. We challenge this assumption and demonstrate that there are a set of conditions defining specific DNA mixtures which can produce an aspirational LR and thereby provide a measure of reproducibility for DNA profiling systems incorporating PG. Such DNA mixtures could serve as the basis for inter-laboratory comparisons, even when different STR amplification kits are employed. We propose a procedure for an inter-laboratory comparison consistent with these conditions.
Highlights
As forensic short tandem repeat (STR) genotyping assays have become more sensitive, DNA samples that may once have been classified as single source may instead be classified as having multiple contributors as low-level alleles are detected
McNevin et al [35] have previously suggested a method for assessing reproducibility and defining credible intervals for likelihood ratios (LRs) derived from the same DNA extracts and calculated by STRmix in particular and continuous probabilistic genotyping (PG) in general
The LR defined by Equation (7) and the LR range defined by Equation (12) and enabled by our conditions 1 to 8 will not depend on either the PG system or the laboratory if each PG system calculates LR according to Equation (5) and calculates wm according to maximum likelihood and if each laboratory has calibrated their PG system appropriately
Summary
As forensic short tandem repeat (STR) genotyping assays have become more sensitive, DNA samples that may once have been classified as single source (assessed as being derived from a single DNA donor) may instead be classified as having multiple contributors as low-level alleles are detected. 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]. We provide a formal proof that such a test exists, and we define the conditions under which such a test could be performed
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