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Complex DNA profile interpretation: stories from across the pond

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FOSW03 - Statistical Modelling of Scientific Evidence

A story of samples and statistics: The history of a forensic sample, the history and current state of forensic DNA interpretation and statistics in the U.S.

With the continued increase in the sensitivity of DNA testing systems comes a commensurate increase in the complexity of the profiles generated. Numerous sophisticated statistical tools intended to provide an appropriate weight of evidence for these challenging samples have emerged over the last several years.  While it seems clear that only a likelihood ratio-based probabilistic genotyping approach is appropriate to address the ambiguity inherent in these complex samples, the relative merits of the different approaches are still being investigated. The first part of this talk will address the generation of DNA samples from a forensic science perspective.  Long before the statistical weight of evidence is considered, numerous decision points determine what samples are collected, what samples are tested, how they are tested and what questions are asked of them.  It is critical to understand the sample history and the milieu in which their journey takes place on their way to becoming profiles that require interpretation and statistical assessment. We will then summarize the history of approaches typically used by working analysts in the US, and discuss the current state of the practice. In 2005 and 2013, NIST distributed sets of mixtures to working laboratories and collected their interpretations and statistical weights. They found a wide range of variation both within and between laboratories in calculating the weight of evidence for the same sample in both surveys.  Most disturbing was the continued use of simplistic tools, such as the CPI /CPE (RMNE), long considered inadequate for specific types of profiles. A number of publications and reports over the last 15 years have commented on the interpretation and statistical weighting of forensic DNA profiles. These include the ISFG commission papers of 2006 and 2012, the NAS 2009 report, the 2010 SWGDAM STR interpretation guidelines, and the 2015 SWGDAM probabilistic genotyping software validation guidelines. Several high profiles criticisms of laboratory protocols (e.g. Washington D.C. and the TX laboratory system) have emerged that have fueled debate. Most recently, PCAST published a report commenting on the state of forensic science disciplines in the US, including DNA . An updated draft of the SWGDAM STR interpretation guidelines is currently posted for comment.  We will discuss these various publications and commentaries as time permits.

This talk is part of the Isaac Newton Institute Seminar Series series.

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