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’Blame the brother’-Assessment of forensic DNA evidence when alternative explanations have different likelihoods
Linköping University, Department of Computer and Information Science, Statistics. Linköping University, Faculty of Arts and Sciences. Polismyndigheten - Nationellt Forensiskt Centrum.
Linköping University, Department of Physics, Chemistry and Biology, Biology. Linköping University, Faculty of Science & Engineering.
Polismyndigheten - Nationellt Forensiskt Centrum.
2011 (English)Conference paper, Poster (Other academic)
Abstract [en]

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.

 

Place, publisher, year, edition, pages
2011.
Keyword [en]
Evidence value, Bayes' factor, Multiple propositions, Scale of conclusions, DNA profiles, Relatives
National Category
Probability Theory and Statistics Law and Society
Identifiers
URN: urn:nbn:se:liu:diva-118554OAI: oai:DiVA.org:liu-118554DiVA: diva2:815484
Conference
ISFG2011. 24th World Congress of the International Society for Forensic Genetics
Available from: 2015-06-01 Created: 2015-06-01 Last updated: 2015-06-10

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