An Information Gap in DNA Evidence Interpretation

Abstract

Forensic DNA evidence often contains mixtures of multiple contributors, or is present in low template amounts. The resulting data signals may appear to be relatively uninformative when interpreted using qualitative inclusion-based methods. However, these same data can yield greater identification information when interpreted by computer using quantitative data-modeling methods. This study applies both qualitative and quantitative interpretation methods to a well-characterized DNA mixture and dilution data set, and compares the inferred match information. The results show that qualitative interpretation loses identification power at low culprit DNA quantities (below 100 pg), but that quantitative methods produce useful information down into the 10 pg range. Thus there is a ten-fold information gap that separates the qualitative and quantitative DNA mixture interpretation approaches. With low quantities of culprit DNA (10 pg to 100 pg), computer-based quantitative interpretation provides greater match sensitivity.

Probabilistic characterisation of baseline noise in STR profiles

“…Interpretation methods that utilise peak height or area information, rather than the binary information of whether a peak height or area exceeds a threshold or not, have garnered much attention, and a variety of fully-continuous interpretation platforms have been developed [9][10][11][12][13][14][15][16].…”

“…In [13], a continuous modelling of the peak height is used if the peak exceeds a user defined analytical threshold. A quantitative computer evaluation of DNA mixtures is proposed in [15,16]. In the latter work, peak heights are modelled as truncated Gaussian random variables and baseline noise is implicitly treated as small peaks with mean zero.…”

The Probabilistic Genotyping Software STRmix: Utility and Evidence for its Validity

“…Peer‐review is another criterion of the Daubert standard that may be considered by the gate keeper. Oxford (https://en.oxforddictionaries.com/definition/peer_review) defines peer‐review as: “Evaluation of scientific, academic, or professional work by others working in the same field.” The scientific concepts that underpin PG software are verified by publication in peer‐reviewed journals and wider comments published elsewhere .…”

Forensic DNA Analysis

“…and the expectation that two heterozygote alleles from the same source should display a similar signal strength for the evaluation of possible genotypes for each component locus by locus. This use of peak height or peak area information for the deconvolution of mixtures was first described by Clayton et al (1998) and has since then been incorporated into interpretation guidelines (Gill et al 2006(Gill et al , 2008 and expert system software (Perlin and Sinelnikov 2009). …”

“…In order to not overestimate the likelihood ratio, thereby possibly incriminating a defendant, it is generally recommended to minimise the number of assumed contributors for the defense hypothesis in the denominator (Brenner et al 1996;Gill et al 2006). An alternative approach for mixture statistics is using expert systems that can model across several possible hypotheses explaining the data (Perlin and Sinelnikov 2009). Refer to Gill and Buckleton (2010) for an overview of concerns regarding low template DNA statistics.…”