Biomedical subjects

Christophe Champod

Publications and source records attributed to Christophe Champod.

9 recordsLinked to original sources

PubMed2007

Value of DNA tests: a decision perspective.

Before a Court of Law testifying in DNA-evidence cases, scientists are often challenged with the idea that the more markers (loci) the better, i.e., why does the scientist not use 16 or more markers? This paper introduces a new perspective, decision analysis, to deal with the problem of the number of markers to type in a criminal context. The decision-making process, which plays a key role in the routine work of a forensic scientist, consists of the rational choice, given personal objectives, between two or more possible outcomes when the consequences of the choice are uncertain. Simulated results support the hypothesis that analytical added value does not increase with the number of markers.

Bayes Theorem↗

PubMed2007

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.

Dermatoglyphics↗

PubMed2006

Evaluation of preliminary isotopic analysis (13C and 15N) of explosives A likelihood ratio approach to assess the links between semtex samples.

Currently, the use of isotopic ratio as corroborative evidence in criminal trials is explored. Beyond the analytical challenges that have been reported elsewhere, the crucial issue of the interpretation of analytical results in a fair and balanced way remains poorly documented. The aim of this paper is to propose a likelihood ratio approach for the evaluation of stable isotope data acquired from semtex samples. It will also lead to recommendations in relation to the acquisition of normalised international data.

Journal Article↗

PubMed2006

Use of gold nanoparticles as molecular intermediates for the detection of fingermarks.

Among the numerous methods dedicated to the detection of latent fingermarks, the MultiMetal Deposition (MMD) offers, as a main advantage, the ability to be applied on a great number of porous and non-porous surfaces, e.g., paper, plastic, glass, latex, and polystyrene, even if wetted. While considered as a powerful and sensitive technique, MMD is often neglected, mainly because of operational limitations (siliconized vessels, restrictive pH domain, numerous immersion baths, ...). In this contribution, we propose a modification of the standard MMD method so that the procedure is simplified with a number of baths reduced to a minimum. To reach this goal, it was necessary to obtain a fully operable solution which could detect fingermarks in a single step. We chose to take advantage of the molecular recognition mechanisms by functionalizing the gold nanoparticles with a molecular host able to bind itself to gold while keeping the ability to trap molecules in solution. Cyclodextrins were chosen as they can be easily chemically modified to offer gold-binding abilities. Moreover, they are widely used as hosts for various molecular guests (dyes, luminescent molecules, ...). This new formulation has been tested on three different surfaces to attest the feasibility of this strategy. Successful results were obtained with detailed fingermarks offering a good contrast to allow their identification without the need to enhance the results (such as with a physical developer). If the new formulation behaves very similarly to the old one, in terms of experimental conditions, it offers the additional advantage to develop fingermarks after immersing them in only one bath. The goal is thus reached.

Cyclodextrins↗

PubMed2006

Evidence evaluation in fingerprint comparison and automated fingerprint identification systems--modelling within finger variability.

Recent challenges and errors in fingerprint identification have highlighted the need for assessing the information content of a papillary pattern in a systematic way. In particular, estimation of the statistical uncertainty associated with this type of evidence is more and more called upon. The approach used in the present study is based on the assessment of likelihood ratios (LRs). This evaluative tool weighs the likelihood of evidence given two mutually exclusive hypotheses. The computation of likelihood ratios on a database of marks of known sources (matching the unknown and non-matching the unknown mark) allows an estimation of the evidential contribution of fingerprint evidence. LRs are computed taking advantage of the scores obtained from an automated fingerprint identification system and hence are based exclusively on level II features (minutiae). The AFIS system attributes a score to any comparison (fingerprint to fingerprint, mark to mark and mark to fingerprint), used here as a proximity measure between the respective arrangements of minutiae. The numerator of the LR addresses the within finger variability and is obtained by comparing the same configurations of minutiae coming from the same source. Only comparisons where the same minutiae are visible both on the mark and on the print are therefore taken into account. The denominator of the LR is obtained by cross-comparison with a database of prints originating from non-matching sources. The estimation of the numerator of the LR is much more complex in terms of specific data requirements than the estimation of the denominator of the LR (that requires only a large database of prints from an non-associated population). Hence this paper addresses specific issues associated with the numerator or within finger variability. This study aims at answering the following questions: (1) how a database for modelling within finger variability should be acquired; (2) whether or not the visualisation technique or the choice of different minutiae arrangements may influence that modelling and (3) what is the magnitude of LRs that can be expected from such a model. Results show that within finger variability is affected by the visualisation technique used on the mark, the number of minutiae and the minutiae configuration. They also show that the rates of misleading evidence in the likelihood ratios obtained for one of the configurations examined are low.

Automation↗

PubMed2006

Multimodal biometrics for identity documents (MBioID).

The MBioID initiative has been set up to address the following germane question: What and how biometric technologies could be deployed in identity documents in the foreseeable future? This research effort proposes to look at current and future practices and systems of establishing and using biometric identity documents (IDs) and evaluate their effectiveness in large-scale developments. The first objective of the MBioID project is to present a review document establishing the current state-of-the-art related to the use of multimodal biometrics in an IDs application. This research report gives the main definitions, properties and the framework of use related to biometrics, an overview of the main standards developed in the biometric industry and standardisation organisations to ensure interoperability, as well as some of the legal framework and the issues associated to biometrics such as privacy and personal data protection. The state-of-the-art in terms of technological development is also summarised for a range of single biometric modalities (2D and 3D face, fingerprint, iris, on-line signature and speech), chosen according to ICAO recommendations and availabilities, and for various multimodal approaches. This paper gives a summary of the main elements of that report. The second objective of the MBioID project is to propose relevant acquisition and evaluation protocols for a large-scale deployment of biometric IDs. Combined with the protocols, a multimodal database will be acquired in a realistic way, in order to be as close as possible to a real biometric IDs deployment. In this paper, the issues and solutions related to the acquisition setup are briefly presented.

Biometry↗

PubMed2006

The potential (negative) influence of observational biases at the analysis stage of fingermark individualisation.

Recent cases of erroneous identification have strengthened critical comments on the reliability of fingerprint identification. This goes hand in hand with recent publications regarding the lack of scientific foundation of the discipline. Combined with "legislative" needs, such as for instance the admissibility criteria under Daubert, or experimental studies revealing potential bias, the call for research on the identification process has become more urgent. That background set the basis of this research project financed by the Swiss National Science Foundation (SNSF) which includes, among other parts, experimental tests to study potential observational biases in the analysis stage of fingerprint individualisation. These tests have been submitted to several groups of forensic science students at the University of Lausanne. The aim is to study factors potentially influencing the analysis of fingermarks, more specifically the influence of training/education (test I) as well as the potential impact of case contextual information or known print availability (test II). For all tests students were given 11 or 12 fingermarks of a medium to difficult quality, with a range of 8-15 minutiae. For all tests the task was always the same for the participants but carried out in different contexts: to analyse the marks, to annotate the minutiae observed, to designate them and to decide on the status of the mark in two categories, exploitable and identifiable. The aim was to see how the fingermarks were annotated by different individuals so as to have an idea of the variation in annotation and counting in the analysis stage only. For test I, students were submitted the same 12 fingerprints before and after having followed specific training in fingermark individualisation. The aim was to see how training/education impacts the analysis of fingermarks. For test II, were participants given eleven fingermarks so as to study whether the presence of a comparison print changes the amount of minutiae found and whether low/high-profile background information influences the analysis stage. Results show that for test I the effect of training can be observed, among other, in an increase of minutiae annotated and a higher consensus between participants. For test II no effect of the stimuli used to induce observational biases has been observed by all of the factors studied.

Dermatoglyphics↗

PubMed2006

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.

Dermatoglyphics↗

PubMed2002

Interpreting small quantities of DNA: the hierarchy of propositions and the use of Bayesian networks.

The dramatic increase in the sensitivity of DNA profiling systems that has occurred over recent years has led to the need to address a wider range of interpretational problems in forensic science. The issues surrounding questions of the kind "whose DNA is this?" have been the subject of considerable controversy but now it is clear that the emphasis is shifting to questions of the kind "how did this DNA get here?" Such issues are discussed in this paper and new insights are provided by two particular recent developments. First, the notion of the "hierarchy of propositions" that has arisen from a project called Case Assessment and Interpretation (CAI) that has been running in the British Forensic Science Service (FSS). Second, a technique for drawing inferences in the face of many interacting considerations, known as "Bayesian networks"--or "Bayes' nets" for short--that has been the subject of an earlier paper in this journal (1). The discussion is carried out by means of case studies, based on actual cases. It is clear that, whereas the inference in relation to the source of the DNA in a crime sample might be overwhelmingly strong, the inference in relation to the propositions that a jury must consider relating to the identity of the actual offender may be much more tentative.

Bayes Theorem↗