Description of the likelihood ratio (LR) changes in STRmix™ v2.4.08

Wednesday, 23 August 2017

We recently made three changes to the likelihood ratio (LR) calculations in STRmix™ resulting in v2.4.08.  This updates the LR calculations in all versions preceding v2.4.08.

1. A change has been made to the LR calculation (unrelated point estimate, stratified, unified and HPD) for mixed DNA profiles when there are multiple contributors considered under Hp who are unknown under Hd.  The contributors must each have dropped alleles (of either the same or different types) at the same locus. 

This change relates to the order of the contributors within the LR calculation.  Before and after LRs from a trial on samples that meet these conditions show that the changes in LR are all small, the largest observed in our trial being an LR of 114 changed to 0.2.  Both these results might be reasonably described as uninformative, hence qualitatively the same.

2. A change has been made to the way drop-in alleles are assigned during the determination of the genotype array within pre burn-in.  This will affect all LR calculations including the database search module (standard and familial LRs), unrelated and related scenarios.  A number of two-person profiles were edited by adding a peak in a double back stutter position of a major contributor’s allele.  The comparison of LRs between versions indicates that this was predominantly within one order of magnitude.   

3. A minor anomaly in the familial search LR was identified.  The issue affected mixed and single source profiles with dropout where on some occasions an incorrect allele frequency was assigned to the alleles.  A comparison of familial LRs calculated using the database search functionality between versions indicates that this was mostly within one order of magnitude.  

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