Caliebe Amke, Walsh Susan, Liu Fan, Kayser Manfred, Krawczak Michael
Institute of Medical Informatics and Statistics, Kiel University, Brunswiker Strasse 10, 24105 Kiel, Germany.
Department of Biology, Indiana-University-Purdue-University-Indianapolis (IUPUI), 723 W. Michigan St. Indianapolis, IN, USA; Department of Genetic Identification, Erasmus MC University Medical Center Rotterdam, PO Box 2040, 3000 CA Rotterdam, The Netherlands.
Forensic Sci Int Genet. 2017 May;28:203-210. doi: 10.1016/j.fsigen.2017.03.004. Epub 2017 Mar 6.
It has become widely accepted in forensics that, owing to a lack of sensible priors, the evidential value of matching DNA profiles in trace donor identification or kinship analysis is most sensibly communicated in the form of a likelihood ratio (LR). This restraint does not abate the fact that the posterior odds (PO) would be the preferred basis for returning a verdict. A completely different situation holds for Forensic DNA Phenotyping (FDP), which is aimed at predicting externally visible characteristics (EVCs) of a trace donor from DNA left behind at the crime scene. FDP is intended to provide leads to the police investigation helping them to find unknown trace donors that are unidentifiable by DNA profiling. The statistical models underlying FDP typically yield posterior odds (PO) for an individual possessing a certain EVC. This apparent discrepancy has led to confusion as to when LR or PO is the appropriate outcome of forensic DNA analysis to be communicated to the investigating authorities. We thus set out to clarify the distinction between LR and PO in the context of forensic DNA profiling and FDP from a statistical point of view. In so doing, we also addressed the influence of population affiliation on LR and PO. In contrast to the well-known population dependency of the LR in DNA profiling, the PO as obtained in FDP may be widely population-independent. The actual degree of independence, however, is a matter of (i) how much of the causality of the respective EVC is captured by the genetic markers used for FDP and (ii) by the extent to which non-genetic such as environmental causal factors of the same EVC are distributed equally throughout populations. The fact that an LR should be communicated in cases of DNA profiling whereas the PO are suitable for FDP does not conflict with theory, but rather reflects the immanent differences between these two forensic applications of DNA information.
在法医学领域，由于缺乏合理的先验概率，人们普遍认为，在微量样本提供者识别或亲缘关系分析中，匹配DNA图谱的证据价值最合理的传达方式是似然比（LR）。但这一限制并未改变后验概率（PO）才是作出裁决的首选依据这一事实。法医DNA表型分析（FDP）则是完全不同的情况，它旨在根据犯罪现场遗留的DNA预测微量样本提供者的外部可见特征（EVC）。FDP旨在为警方调查提供线索，帮助他们找到无法通过DNA图谱识别的未知微量样本提供者。FDP所基于的统计模型通常会得出个体具有某种EVC的后验概率（PO）。这种明显的差异导致了在向调查当局传达法医DNA分析的适当结果时，究竟是LR还是PO存在混淆。因此，我们从统计学角度出发，着手阐明法医DNA图谱分析和FDP中LR与PO的区别。在此过程中，我们还探讨了群体归属对LR和PO的影响。与DNA图谱分析中LR众所周知的群体依赖性不同，FDP中获得的PO可能在很大程度上与群体无关。然而，实际的独立程度取决于以下两个方面：（i）用于FDP的基因标记能够捕捉到相应EVC因果关系的程度；（ii）同一EVC的非遗传因果因素（如环境因素）在各群体中分布的均匀程度。在DNA图谱分析的情况下应传达LR，而PO适用于FDP，这一事实与理论并不冲突，而是反映了DNA信息这两种法医应用之间的内在差异。
