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The effect of a user selected number of contributors within the LR assignment

NCJ Number
Australian Journal of Forensic Sciences Volume: 54 Issue: 4 Dated: 2022 Pages: 450-463
Hannah Kelly; Jo-Anne Bright; Maarten Kruijver; Duncan Taylor; John Buckleton
Date Published
14 pages

This article presents research into the assignment of the number of contributors within a forensic DNA profile, and the effect of this process when both legal prosecution and defense present contradictory interpretations.


The assignment of the number of contributors (N) to a forensic DNA profile is undertaken as part of the interpretation process. There is no requirement for N to be the same for both propositions within the likelihood ratio framework. ISFG recommendations on mixture interpretation suggest that there may be times where prosecution and defense both specify their own, different N. The authors investigate how this affects the likelihood ratio (LR) for 100 mixed DNA profiles. The authors show that the addition of a superfluous unknown contributor within a proposition tends to increase the likelihood because the profile can always be better described with more contributors but the priors on the additional parameters and an extra genotype can ultimately reduce the likelihood ratio. Additionally, the authors find that choosing improbable values for N given different propositions can lead to misleading LRs. Specifically, the authors find that LRs can both increase and decrease compared to the value obtained using the ground truth N. Choosing a single N which maximizes the probability of the observations for each party tends to approximate an exhaustive stratified LR that takes into account different Ns with different prior probabilities. (Published Abstract Provided)