DNA commission of the International society for forensic genetics: Assessing the value of forensic biological evidence - Guidelines highlighting the importance of propositions. Part II: Evaluation of biological traces considering activity level propositions

Abstract

The value of the evidence depends critically on propositions. In the second of two papers intended to provide advice to the community on difficult aspects of evaluation and the formulation of propositions, we focus primarily on activity level propositions. This helps the court address the question of "How did an individual's cell material get there?". In order to do this, we expand the framework outlined in the first companion paper. First, it is important not to conflate results and propositions. Statements given activity level propositions aim to help address issues of indirect vs direct transfer, and the time of the activity, but it is important to avoid use of the word 'transfer' in propositions. This is because propositions are assessed by the Court, but DNA transfer is a factor that scientists need to take into account for the interpretation of their results. Suitable activity level propositions are ideally set before knowledge of the results and address issues like: X stabbed Y vs. an unknown person stabbed Y but X met Y the day before. The scientist assigns the probability of the evidence, if each of the alternate propositions is true, to derive a likelihood ratio. To do this, the scientist asks: a) "what are the expectations if each of the propositions is true?" b) "What data are available to assist in the evaluation of the results given the propositions?" When presenting evidence, scientists work within the hierarchy of propositions framework. The value of evidence calculated for a DNA profile cannot be carried over to higher levels in the hierarchy -the calculations given sub-source, source and activity level propositions are all separate. A number of examples are provided to illustrate the principles espoused, and the criteria that such assessments should meet. Ideally in order to assign probabilities, the analyst should have/collect data that are relevant to the case in

DNA detection of a temporary and original user of an office space

“…As such, these probabilities can be used to devise a sampling strategy to: (a) target areas to detect DNA relevant to the case, or (b) identify control locations in scenarios of legitimate interaction with an environment. The assigned probabilities can also be used to interpret the findings and assign a weight to the evidence given activity level propositions [36]. In the absence of further studies of other environments, this data may also be useful to extrapolate in respect to other similar spaces, such as someone's home or car.…”

AnLRframework incorporating sensitivity analysis to model multiple direct and secondary transfer events on skin surface

“…Mr. X and Ms. Y had social interaction and she was assaulted by him at the times specified 2. Mr. X and Ms Y only had social interaction at the times specified Note that to consider activity level propositions, the court would need to accept that Mr. Y was a donor to the DNA sample which was recovered, in accordance with ISFG DNA commission recommendations [1]. The subsource LR φ cannot be used to inform the value of the evidence at activity level.…”

Digital evidence exceptionalism? A review and discussion of conceptual hurdles in digital evidence transformation

“…as proposed in Ref. [ 68 , 80 ], start from numbers and relate them to words, not the reverse [ 7 , 42 , 67 ]. DECDs, in turn, start and end with words.…”