Effects of prescreening for likelihood ratio approaches in the forensic identification of source problems

Source prescreening is a methodology where forensic examiners select samples that are similar to given trace evidence to represent the background population. This background evidence helps assign a value of evidence using a likelihood ratio or Bayes factor. A potential benefit of prescreening is a mitigation of effects from subpopulation structures within the alternative source population by isolating the relevant subpopulation. This paper examines the impact of prescreening before assigning evidence value. Extensive simulations with synthetic and real data, including trace element and fingerprint score examples, were conducted. The findings indicate that when the subpopulation structure is known, prescreening can provide an accurate evidence value in cases of subpopulation structures. The study suggests that prescreening can be beneficial, but the prescreening method and level should be transparently reported alongside the evidence value.