Resolving paternity relationships using X-chromosome STRs and Bayesian networks

X-chromosomal short tandem repeats (X-STRs) are very useful in complex paternity cases because they are inherited by male and female offspring in different ways. They complement autosomal STRs (as-STRs) allowing higher paternity probabilities to be attained. These probabilities are expressed in a likelihood ratio (LR). The formulae needed to calculate LR depend on the genotype combinations of suspected pedigrees. LR can also be obtained by the use of Bayesian networks (BNs). These are graphical representations of real situations that can be used to easily calculate complex probabilities. In the present work, two BNs are presented, which are designed to derive LRs for half-sisters/half-sisters and mother/daughter/paternal grandmother relationships. These networks were validated against known formulae and show themselves to be useful in other suspect pedigree situations than those for which they were developed. The BNs were applied in two paternity cases. The application of the mother/daughter/paternal grandmother BN highlighted the complementary value of X-STRs to as-STRs. The same case evaluated without the mother underlined that missing information tends to be conservative if the alleged father is the biological father and otherwise nonconservative. The half-sisters case shows a limitation of statistical interpretations in regard to high allelic frequencies.     

Inclusion Probabilities and Dropout

Abstract: Recent discussions on a forensic discussion group highlighted the prevalence of a practice in the application of inclusion probabilities when dropout is possible that is of significant concern. In such cases, there appears to be an unpublished practice of calculation of an inclusion probability only for those loci at which the profile of interest (hereafter the suspect) is fully included among the alleles present in the crime scene sample and to omit those loci at which the suspect has alleles that are not fully represented among the alleles in the mixture. The danger is that this approach may produce apparently strong evidence against a surprisingly large fraction of noncontributors. In this paper, the risk associated with the approach of ignoring loci with discordant alleles is assessed by simulation.     

Bayesian networks and the value of the evidence for the forensic two-trace transfer problem*

Forensic scientists face increasingly complex inference problems for evaluating likelihood ratios (LRs) for an appropriate pair of propositions. Up to now, scientists and statisticians have derived LR formulae using an algebraic approach. However, this approach reaches its limits when addressing cases with an increasing number of variables and dependence relationships between these variables. In this study, we suggest using a graphical approach, based on the construction of Bayesian networks (BNs). We first construct a BN that captures the problem, and then deduce the expression for calculating the LR from this model to compare it with existing LR formulae. We illustrate this idea by applying it to the evaluation of an activity level LR in the context of the two-trace transfer problem. Our approach allows us to relax assumptions made in previous LR developments, produce a new LR formula for the two-trace transfer problem and generalize this scenario to n traces.     

Sampling and statistical considerations for the Suchey–Brooks method for pubic bone age estimation: Implications for regional comparisons

Although the Suchey–Brooks (SB) system is currently the most widely used method for age-at-death estimation from the pubic bone, the system continues to evolve through stepwise improvements. Since the system was developed from a pubic bone sample derived mainly from North Americans, it is unclear how well it performs on populations from other continents. During the last decade, studies of the SB system on pubic bone samples from local populations in Europe and Asia have indicated regional differences in the relationship between age and pubic bone development. However, these studies have for the most part followed different research protocols, which make comparisons between their results less meaningful. It would be most useful if future regional analysis of the SB system were done in a rigorous and uniform fashion, following standard procedures. In this paper, sampling and statistical considerations are outlined that hopefully will help to standardize research on the SB system.     

Computation of likelihood ratios in fingerprint identification for configurations of any number of minutiae

Recent court challenges have highlighted the need for statistical research on fingerprint identification. This paper proposes a model for computing likelihood ratios (LRs) to assess the evidential value of comparisons with any number of minutiae. The model considers minutiae type, direction and relative spatial relationships. It expands on previous work on three minutiae by adopting a spatial modeling using radial triangulation and a probabilistic distortion model for assessing the numerator of the LR. The model has been tested on a sample of 686 ulnar loops and 204 arches. Features vectors used for statistical analysis have been obtained following a preprocessing step based on Gabor filtering and image processing to extract minutiae data. The metric used to assess similarity between two feature vectors is based on an Euclidean distance measure. Tippett plots and rates of misleading evidence have been used as performance indicators of the model. The model has shown encouraging behavior with low rates of misleading evidence and a LR power of the model increasing significantly with the number of minutiae. The LRs that it provides are highly indicative of identity of source on a significant proportion of cases, even when considering configurations with few minutiae. In contrast with previous research, the model, in addition to minutia type and direction, incorporates spatial relationships of minutiae without introducing probabilistic independence assumptions. The model also accounts for finger distortion.     

Computation of likelihood ratios in fingerprint identification for configurations of three minutiae

Recent challenges to fingerprint evidence have brought forward the need for peer-reviewed scientific publications to support the evidential value assessment of fingerprint. This paper proposes some research directions to gather statistical knowledge of the within-source and between-sources variability of configurations of three minutiae on fingermarks and fingerprints. This paper proposes the use of the likelihood ratio (LR) approach to assess the value of fingerprint evidence. The model explores the statistical contribution of configurations of three minutiae using Tippett plots and related measures to assess the quality of the system. Features vectors used for statistical analysis have been obtained following a preprocessing step based on Gabor filtering and image processing to extract minutia position, type, and direction. Spatial relationships have been coded using Delaunay triangulation. The metric, used to assess similarity between two feature vectors is based on an Euclidean distance measure. The within-source variability has been estimated using a sample of 216 fingerprints from four fingers (two donors). Between-sources variability takes advantage of a database of 818 ulnar loops from randomly selected males. The results show that the data-driven approach adopted here is robust. The magnitude of LRs obtained under the prosecution and defense propositions stresses upon the major evidential contribution that small portions of fingermark, containing three minutiae, can provide regardless of its position on the general pattern.     

刑事鉴定制度修改的背景、争议及解读

《刑事诉讼法》在鉴定问题上进行了大幅度地修改,改变了鉴定作为证据种类的名称,删除了省级人民政府指定的医院的规定,增加了对鉴定人出庭作证及其人身权利保障等内容,其进步意义应当是肯定的。然而,《刑事诉讼法》有关鉴定修改触及的内容均需要从修改的缘故、争议问题被修改的程度以及执行中可能遇到的障碍等方面予以理论分析与诠释,以便对此问题有一个科学的认识与理解。     

Prediction of eye color from genetic data using Bayesian approach

Prediction of visible traits from genetic data in certain forensic cases may provide important information that can speed up the process of investigation. Research that has been conducted on the genetics of pigmentation has revealed polymorphisms that explain a significant proportion of the variation observed in human iris color. Here, on the basis of genetic data for the six most relevant eye color predictors, two alternative Bayesian network model variants were developed and evaluated for their accuracy in prediction of eye color. The first model assumed eye color to be categorized into blue, brown, green, and hazel, while the second variant assumed a simplified classification with two states: light and dark. It was found that particularly high accuracy was obtained for the second model, and this proved that reliable differentiation between light and dark irises is possible based on analysis of six single nucleotide polymorphisms and a Bayesian procedure of evidence interpretation.     

Questions,propositions and assessing different levels of evidence: Forensic voice comparison in practice

This paper contributes to the ongoing discussion about the distinction between observations and propositions in forensic inference, with a specific focus on forensic voice comparison casework conducted in the UK. We outline both linguistic and legal issues which make the evaluation of voice evidence and the refinement of propositions problematic in practice, and illustrate these using case examples. We will argue that group-level observations from the offender sample will always be evidential and that the value of this evidence must be determined by the expert. As such, a proposal is made that experts should, at least conceptually, think of voice evidence as having two levels, both with evidential value: group-level and individual-level. The two rely on different underlying assumptions, and the group-level observations can be used to inform the individual-level propositions. However, for the sake of interpretability, it is probably preferable to present only one combined conclusion to the end user. We also wish to reiterate points made in previous work: in providing conclusions, the forensic expert must acknowledge that the value of the evidence is dependent on a number of assumptions (propositions and background information) and these assumptions must be made clear and explicit to the user.     

Consensus on validation of forensic voice comparison

Since the 1960s, there have been calls for forensic voice comparison to be empirically validated under casework conditions. Since around 2000, there have been an increasing number of researchers and practitioners who conduct forensic-voice-comparison research and casework within the likelihood-ratio framework. In recent years, this community of researchers and practitioners has made substantial progress toward validation under casework conditions becoming a standard part of practice: Procedures for conducting validation have been developed, along with graphics and metrics for representing the results, and an increasing number of papers are being published that include empirical validation of forensic-voice-comparison systems under conditions reflecting casework conditions. An outstanding question, however, is: In the context of a case, given the results of an empirical validation of a forensic-voice-comparison system, how can one decide whether the system is good enough for its output to be used in court? This paper provides a statement of consensus developed in response to this question. Contributors included individuals who had knowledge and experience of validating forensic-voice-comparison systems in research and/or casework contexts, and individuals who had actually presented validation results to courts. They also included individuals who could bring a legal perspective on these matters, and individuals with knowledge and experience of validation in forensic science more broadly. We provide recommendations on what practitioners should do when conducting evaluations and validations, and what they should present to the court. Although our focus is explicitly on forensic voice comparison, we hope that this contribution will be of interest to an audience concerned with validation in forensic science more broadly. Although not written specifically for a legal audience, we hope that this contribution will still be of interest to lawyers.     

Validating TrueAllele® DNA mixture interpretation

Abstract: DNA mixtures with two or more contributors are a prevalent form of biological evidence. Mixture interpretation is complicated by the possibility of different genotype combinations that can explain the short tandem repeat (STR) data. Current human review simplifies this interpretation by applying thresholds to qualitatively treat STR data peaks as all‐or‐none events and assigning allele pairs equal likelihood. Computer review, however, can work instead with all the quantitative data to preserve more identification information. The present study examined the extent to which quantitative computer interpretation could elicit more identification information than human review from the same adjudicated two‐person mixture data. The base 10 logarithm of a DNA match statistic is a standard information measure that permits such a comparison. On eight mixtures having two unknown contributors, we found that quantitative computer interpretation gave an average information increase of 6.24 log units (min = 2.32, max = 10.49) over qualitative human review. On eight other mixtures with a known victim reference and one unknown contributor, quantitative interpretation averaged a 4.67 log factor increase (min = 1.00, max = 11.31) over qualitative review. This study provides a general treatment of DNA interpretation methods (including mixtures) that encompasses both quantitative and qualitative review. Validation methods are introduced that can assess the efficacy and reproducibility of any DNA interpretation method. An in‐depth case example highlights 10 reasons (at 10 different loci) why quantitative probability modeling preserves more identification information than qualitative threshold methods. The results validate TrueAllele^® DNA mixture interpretation and establish a significant information improvement over human review.     

谁是司法鉴定的“守门人”？——《关于司法鉴定管理问题的决定》实施五周年成效评析

本文从司法鉴定机构设置、司法鉴定活动启动以及司法鉴定意见审查三个方面论述了司法鉴定“守门人”问题。通过对两大法系国家各自司法鉴定制度的分析,梳理司法鉴定“守门人”的三道防线。同时有针对性地比较和讨论了我国司法鉴定制度缺陷,明确司法鉴定制度改革不能以牺牲鉴定意见“守门人”功能为代价。指出建立国家所属的具有公益性、中立性的司法鉴定机构是发展的必由之路。