Evaluation of Forensic DNA Traces When Propositions of Interest Relate to Activities: Analysis and Discussion of Recurrent Concerns

Abstract

When forensic scientists evaluate and report on the probative strength of single DNA traces, they commonly rely on only one number, expressing the rarity of the DNA profile in the population of interest. This is so because the focus is on propositions regarding the source of the recovered trace material, such as “the person of interest is the source of the crime stain.” In particular, when the alternative proposition is “an unknown person is the source of the crime stain,” one is directed to think about the rarity of the profile. However, in the era of DNA profiling technology capable of producing results from small quantities of trace material (i.e., non-visible staining) that is subject to easy and ubiquitous modes of transfer, the issue of source is becoming less central, to the point that it is often not contested. There is now a shift from the question “whose DNA is this?” to the question “how did it get there?” As a consequence, recipients of expert information are now very much in need of assistance with the evaluation of the meaning and probative strength of DNA profiling results when the competing propositions of interest refer to different activities. This need is widely demonstrated in day-to-day forensic practice and is also voiced in specialized literature. Yet many forensic scientists remain reluctant to assess their results given propositions that relate to different activities. Some scientists consider evaluations beyond the issue of source as being overly speculative, because of the lack of relevant data and knowledge regarding phenomena and mechanisms of transfer, persistence and background of DNA. Similarly, encouragements to deal with these activity issues, expressed in a recently released European guideline on evaluative reporting (Willis et al., 2015), which highlights the need for rethinking current practice, are sometimes viewed skeptically or are not considered feasible. In this discussion paper, we select and discuss recurrent skeptical views brought to our attention, as well as some of the alternative solutions that have been suggested. We will argue that the way forward is to address now, rather than later, the challenges associated with the evaluation of DNA results (from small quantities of trace material) in light of different activities to prevent them being misrepresented in court.

“Source” or “Activity” What is the Level of Issue in a Criminal Trial?

“…This overvaluing has its root cause in the presentation, without qualification, of match probabilities, or LRs, for matching profiles in the context of what have been called issues of “source”. However, if the fact at issue is more appropriately at the level of “activity”, then the probative value of the evidence may be much lower 5,6 …”

Bayesian networks for the interpretation of biological evidence

“…The hierarchy of propositions is a useful mechanism for considering forensic evidence and its context in relation to different activities that may be proposed during the course of an investigation or at trial (Biedermann et al, ; Buckleton et al, ; Cook, Evett, Jackson, Jones, & Lambert, ; Evett, Gill, Jackson, Whitaker, & Champod, ; Jackson, Jones, Booth, Champod, & Evett, ; Kokshoorn et al, ; Taylor, Abarno, et al, ). The concept was first formed during a project carried out by the Forensic Science Service in the United Kingdom, called case assessment and interpretation (Cook et al, ).…”

“…An alternative approach, sourcing data from the literature is a possibility providing that the data can be found. The idea of systematically organizing data such as research into the transfer and persistence of different evidence types is not new (Biedermann et al, ; Buckleton & Walsh, ; Evett, ; Szkuta, Ballantyne, Kokshoorn, & van Oorschot, ), but centralized open source resources are still not readily available. Such a repository would allow greater sharing of knowledge as well as more consistency between scientists.…”

“…When experts do not agree on a particular probability or probabilities, or if there is a range of possible values, sensitivity analysis is a method that can be used to determine how sensitive the LR is to changes in that particular probability (Aitken et al, ; Aitken & Gammerman, ; Biedermann et al, ; Biedermann & Taroni, ; Taylor, Hicks, & Champod, ; Taylor, Kokshoorn, & Biedermann, ). A range of possible values can be used and if the effect on the LR is significant, then either more research is necessary or the network may not be appropriate.…”

“…There has been a reluctance to use Bayesian networks to interpret biological evidence by forensic scientists (Biedermann et al, ; Taylor, Kokshoorn, & Biedermann, ). This is different to distrust of Bayesian methods more generally, for example, the use of the likelihood ratios to evaluate sub‐source (DNA) evidence.…”