| 1. |
- Ahlinder, Jon, et al.
(författare)
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Chemometrics comes to court: evidence evaluation of chem–bio threat agent attacks
- 2015
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Ingår i: Journal of Chemometrics. - : John Wiley & Sons. - 0886-9383 .- 1099-128X. ; 29:5, s. 267-276
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Tidskriftsartikel (refereegranskat)abstract
- Forensic statistics is a well-established scientific field whose purpose is to statistically analyze evidence in order to support legal decisions. It traditionally relies on methods that assume small numbers of independent variables and multiple samples. Unfortunately, such methods are less applicable when dealing with highly correlated multivariate data sets such as those generated by emerging high throughput analytical technologies. Chemometrics is a field that has a wealth of methods for the analysis of such complex data sets, so it would be desirable to combine the two fields in order to identify best practices for forensic statistics in the future. This paper provides a brief introduction to forensic statistics and describes how chemometrics could be integrated with its established methods to improve the evaluation of evidence in court.The paper describes how statistics and chemometrics can be integrated, by analyzing a previous know forensic data set composed of bacterial communities from fingerprints. The presented strategy can be applied in cases where chemical and biological threat agents have been illegally disposed.
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| 2. |
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| 3. |
- Ansell, Ricky, et al.
(författare)
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Interpretation of DNA Evidence: Implications of Thresholds Used in the Forensic Laboratory
- 2014
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Konferensbidrag (övrigt vetenskapligt/konstnärligt)abstract
- Evaluation of forensic evidence is a process lined with decisions and balancing, not infrequently with a substantial deal of subjectivity. Already at the crime scene a lot of decisions have to be made about search strategies, the amount of evidence and traces recovered, later prioritised and sent further to the forensic laboratory etc. Within the laboratory there must be several criteria (often in terms of numbers) on how much and what parts of the material should be analysed. In addition there is often a restricted timeframe for delivery of a statement to the commissioner, which in reality might influence on the work done. The path of DNA evidence from the recovery of a trace at the crime scene to the interpretation and evaluation made in court involves several decisions based on cut-offs of different kinds. These include quality assurance thresholds like limits of detection and quantitation, but also less strictly defined thresholds like upper limits on prevalence of alleles not observed in DNA databases. In a verbal scale of conclusions there are lower limits on likelihood ratios for DNA evidence above which the evidence can be said to strongly support, very strongly support, etc. a proposition about the source of the evidence. Such thresholds may be arbitrarily chosen or based on logical reasoning with probabilities. However, likelihood ratios for DNA evidence depend strongly on the population of potential donors, and this may not be understood among the end-users of such a verbal scale. Even apparently strong DNA evidence against a suspect may be reported on each side of a threshold in the scale depending on whether a close relative is part of the donor population or not. In this presentation we review the use of thresholds and cut-offs in DNA analysis and interpretation and investigate the sensitivity of the final evaluation to how such rules are defined. In particular we show what are the effects of cut-offs when multiple propositions about alternative sources of a trace cannot be avoided, e.g. when there are close relatives to the suspect with high propensities to have left the trace. Moreover, we discuss the possibility of including costs (in terms of time or money) for a decision-theoretic approach in which expected values of information could be analysed.
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| 4. |
- Bovens, Michael, et al.
(författare)
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Chemometrics in forensic chemistry — Part I: Implications to the forensic workflow
- 2019
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Ingår i: Forensic Science International. - : Elsevier BV. - 0379-0738 .- 1872-6283. ; 301, s. 82-90
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Tidskriftsartikel (refereegranskat)abstract
- The forensic literature shows a clear trend towards increasing use of chemometrics (i.e. multivariate analysis and other statistical methods). This can be seen in different disciplines such as drug profiling, arson debris analysis, spectral imaging, glass analysis, age determination, and more. In particular, current chemometric applications cover low-dimensional (e.g. drug impurity profiles) and high-dimensional data (e.g. Infrared and Raman spectra) and are therefore useful in many forensic disciplines. There is a dominant and increasing need in forensic chemistry for reliable and structured processing and interpretation of analytical data. This is especially true when classification (grouping) or profiling (batch comparison) is of interest.Chemometrics can provide additional information in complex crime cases and enhance productivity by improving the processes of data handling and interpretation in various applications. However, the use of chemometrics in everyday work tasks is often considered demanding by forensic scientists and, consequently, they are only reluctantly used. This article and following planned contributions are dedicated to those forensic chemists, interested in applying chemometrics but for any reasons are limited in the proper application of statistical tools — usually made for professionals — or the direct support of statisticians. Without claiming to be comprehensive, the literature reviewed revealed a sufficient overview towards the preferably used data handling and chemometric methods used to answer the forensic question. With this basis, a software tool will be designed (part of the EU project STEFA-G02) and handed out to forensic chemist with all necessary elements of data handling and evaluation.Because practical casework is less and less accompanied from the beginning to the end out of the same hand, more and more interfaces are built in through specialization of individuals. This article presents key influencing elements in the forensic workflow related to the most meaningful chemometric application and evaluation.
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| 5. |
- Lindgren, Petter, et al.
(författare)
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A likelihood ratio-based approach for improved source attribution in microbiological forensic investigations
- 2019
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Ingår i: Forensic Science International. - : Elsevier. - 0379-0738 .- 1872-6283. ; 302
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Tidskriftsartikel (refereegranskat)abstract
- A common objective in microbial forensic investigations is to identify the origin of a recovered pathogenic bacterium by DNA sequencing. However, there is currently no consensus about how degrees of belief in such origin hypotheses should be quantified, interpreted, and communicated to wider audiences. To fill this gap, we have developed a concept based on calculating probabilistic evidential values for microbial forensic hypotheses. The likelihood-ratio method underpinning this concept is widely used in other forensic fields, such as human DNA matching, where results are readily interpretable and have been successfully communicated in juridical hearings. The concept was applied to two case scenarios of interest in microbial forensics: (1) identifying source cultures among series of very similar cultures generated by parallel serial passage of the Tier 1 pathogen Francisella tularensis, and (2) finding the production facilities of strains isolated in a real disease outbreak caused by the human pathogen Listeria monocytogenes. Evidence values for the studied hypotheses were computed based on signatures derived from whole genome sequencing data, including deep-sequenced low-frequency variants and structural variants such as duplications and deletions acquired during serial passages. In the F. tularensis case study, we were able to correctly assign fictive evidence samples to the correct culture batches of origin on the basis of structural variant data. By setting up relevant hypotheses and using data on cultivated batch sources to define the reference populations under each hypothesis, evidential values could be calculated. The results show that extremely similar strains can be separated on the basis of amplified mutational patterns identified by high-throughput sequencing. In the L. monocytogenes scenario, analyses of whole genome sequence data conclusively assigned the clinical samples to specific sources of origin, and conclusions were formulated to facilitate communication of the findings. Taken together, these findings demonstrate the potential of using bacterial whole genome sequencing data, including data on both low frequency SNP signatures and structural variants, to calculate evidence values that facilitate interpretation and communication of the results. The concept could be applied in diverse scenarios, including both epidemiological and forensic source tracking of bacterial infectious disease outbreaks.
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| 6. |
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| 7. |
- Nordgaard, Anders, 1962-, et al.
(författare)
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Assessment of forensic findings when alternative explanations have different likelihoods—“Blame-the-brother”-syndrome
- 2012
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Ingår i: Science & justice. - : Elsevier. - 1355-0306 .- 1876-4452. ; 52:4, s. 226-236
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Tidskriftsartikel (refereegranskat)abstract
- Assessment of forensic findings with likelihood ratios is for several cases straightforward, but there are a number of situations where contemplation of the alternative explanation to the evidence needs consideration, in particular when it comes to the reporting of the evidentiary strength. The likelihood ratio approach cannot be directly applied to cases where the proposition alternative to the forwarded one is a set of multiple propositions with different likelihoods and different prior probabilities. Here we present a general framework based on the Bayes' factor as the quantitative measure of evidentiary strength from which it can be deduced whether the direct application of a likelihood ratio is reasonable or not. The framework is applied on DNA evidence in forms of an extension to previously published work. With the help of a scale of conclusions we provide a solution to the problem of communicating to the court the evidentiary strength of a DNA match when a close relative to the suspect has a non-negligible prior probability of being the source of the DNA.
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| 8. |
- Nordgaard, Anders, 1962-, et al.
(författare)
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’Blame the brother’-Assessment of forensic DNA evidence when alternative explanations have different likelihoods
- 2011
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Ingår i: Book of Abstracts. ; , s. 196-
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Konferensbidrag (övrigt vetenskapligt/konstnärligt)abstract
- In a crime case where a suspect is assumed to be the donor of a recovered stain, forensic DNA evidence presented in terms of a likelihood ratio is a clear course as long as the set of alternative donors contains no close relative of the suspect, since the latter has a higher likelihood than has an individual unrelated to the suspect. The state-of-art today at several laboratories is to report the likelihood ratio but with a reservation stating its lack of validity if the stain originates from a close relative. Buckleton et al[†] derived a so-called extended likelihood ratio for reporting DNA evidence values when a full sibling is present in the set of potential alternative donors. This approach requires consideration of prior probabilities for each of the alternative donors to be the source of the stain and may therefore be problematic to apply in practice. Here we present an alternative way of using prior probabilities in the extended likelihood ratio when the latter is reported on an ordinal scale of conclusions. Our example show that for a 12 STR-marker profile using the extended likelihood ratio approach would not imply a change in the level reported compared to the ordinary likelihood ratio approach, unless the close relative has a very high prior probability of being the donor compared to an unrelated individual.[†] Buckleton JS, Triggs CM, Champod C., Science & Justice 46: 69-78.
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| 9. |
- Nordgaard, Anders, 1962-
(författare)
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Classification of percentages in seizures of narcotic material
- 2017
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Konferensbidrag (övrigt vetenskapligt/konstnärligt)abstract
- The percentage of the narcotic substance in a drug seizure may vary a lot depending on when and from whom the seizure was taken. Seizures from a typical consumer would in general show low percentages, while seizures from the early stages of a drug dealing chain would show higher percentages (these will be diluted). Legal fact finders must have an up-to-date picture of what is an expected level of the percentage and what levels are to be treated as unusually low or unusually high. This is important for the determination of the sentences to be given in a drug case.In this work we treat the probability distribution of the percentage of a narcotic substance in a seizure from year to year as a time series of beta density functions, which are successively updated with the use of point mass posteriors for the shape parameters. The predictive distribution for a new year is a weighted sum of beta distributions for the previous years where the weights are found from forward validation. We show that this method of prediction is more accurate than one that uses a predictive distribution built on a likelihood based on all previous years.
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| 10. |
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