Cedric Neumann

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

ABSTRACT: 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 minutiæ. 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 minutiæ. The LRs that it provides are highly indicative of identity of source on a significant proportion of cases, even when considering configurations with few minutiæ. In contrast with previous research, the model, in addition to minutia type and direction, incorporates spatial relationships of minutiæ without introducing probabilistic independence assumptions. The model also accounts for finger distortion.

Interpretation of complex DNA profiles using empirical models and a method to measure their robustness

A new methodology is presented in order to report complex DNA profiles. We have brought together a number of different theories in order to devise a new protocol to interpret complex cases using likelihood ratios. The calculations are designed to be highly conservative and are widely applicable. We apply a low copy number (LCN) interpretation framework, which includes the probabilities of dropout and contamination, to conventional DNA cases. In conventional casework, stutters often compromise calculations when they are observed with the same height as a minor contributor to a mixture. Stutters cannot be distinguished from minor alleles. We compensate by treating them as real alleles and including them in the calculation. By increasing the number of potential contributors to the DNA profile, we can account for the extra alleles that result. We propose that the likelihood ratio is qualified with additional robustness parameters to indicate the probability of misleading evidence in favour of the prosecution, under the assumption that a random man was a contributor instead of the suspect. To do this we apply a new kind of case-specific Tippett test. Although the method is complex, we suggest a user-friendly way to explain the results to a court. The method is easily extended to carry out ranked likelihood ratio (LR) searches for suspects in national DNA databases.

Computation of Likelihood Ratios in Fingerprint Identification for Configurations of Three Minutiae

ABSTRACT: 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 minutiæ 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 minutiæ, can provide regardless of its position on the general pattern.

Considerations on the ASTM standards 1789-04 and 1422-05 on the forensic examination of ink.

Abstract:u2002 The ASTM standards on Writing Ink Identification (ASTM 1789‐04) and on Writing Ink Comparison (ASTM 1422‐05) are the most up‐to‐date guidelines that have been published on the forensic analysis of ink. The aim of these documents is to cover most aspects of the forensic analysis of ink evidence, from the analysis of ink samples, the comparison of the analytical profile of these samples (with the aim to differentiate them or not), through to the interpretation of the result of the examination of these samples in a forensic context. Significant evolutions in the technology available to forensic scientists, in the quality assurance requirements brought onto them, and in the understanding of frameworks to interpret forensic evidence have been made in recent years. This article reviews the two standards in the light of these evolutions and proposes some practical improvements in terms of the standardization of the analyses, the comparison of ink samples, and the interpretation of ink examination. Some of these suggestions have already been included in a DHS funded project aimed at creating a digital ink library for the United States Secret Service.