Using graphical probability analysis (Bayes Nets) to evaluate a conditional DNA inclusion

This paper discusses the analysis of cases in which the inclusion or exclusion of a particular suspect, as a possible contributor to a DNA mixture, depends on the value of a variable (the number of contributors) that cannot be determined with certainty. It offers alternative ways to deal with such cases, including sensitivity analysis and object-oriented Bayesian networks, that separate uncertainty about the inclusion of the suspect from uncertainty about other variables. The paper presents a case study in which the value of DNA evidence varies radically depending on the number of contributors to a DNA mixture: if there are two contributors, the suspect is excluded; if there are three or more, the suspect is included; but the number of contributors cannot be determined with certainty. It shows how an object-oriented Bayesian network can accommodate and integrate varying perspectives on the unknown variable and how it can reduce the potential for bias by directing attention to relevant considerations and distinguishing different sources of uncertainty. It also discusses the challenge of presenting such evidence to lay audiences.

[1]  J. Koehler,et al.  The Psychology of Numbers in the Courtroom: How to Make DNA Match Statistics Seem Impressive or Insufficient , 2001 .

[2]  J S Buckleton,et al.  Setting bounds for the likelihood ratio when multiple hypotheses are postulated. , 1998, Science & justice : journal of the Forensic Science Society.

[3]  J Mortera,et al.  Object-oriented Bayesian networks for complex forensic DNA profiling problems. , 2007, Forensic science international.

[4]  Dw Van Boxel,et al.  Probabilistic Expert Systems for Forensic Inference from Genetic Markers , 2002 .

[5]  Peter Gill,et al.  Towards understanding the effect of uncertainty in the number of contributors to DNA stains. , 2007, Forensic science international. Genetics.

[6]  J. Buckleton,et al.  Forensic DNA Evidence Interpretation , 2004 .

[7]  William C. Thompson,et al.  What role should investigative facts play in the evaluation of scientific evidence , 2011 .

[8]  Wing K. Fung,et al.  The statistical evaluation of DNA mixtures with contributors from different ethnic groups , 2009, International Journal of Legal Medicine.

[9]  William C. Thompson,et al.  Painting the target around the matching profile: the Texas sharpshooter fallacy in forensic DNA interpretation , 2009 .

[10]  Dale A. Nance,et al.  Juror Understanding of DNA Evidence: An Empirical Assessment of Presentation Formats for Trace Evidence with a Relatively Small Random‐Match Probability , 2005, The Journal of Legal Studies.

[11]  Peter M Vallone,et al.  Allele frequencies for 15 autosomal STR loci on U.S. Caucasian, African American, and Hispanic populations. , 2003, Journal of forensic sciences.

[12]  Bruce S. Weir,et al.  DNA statistics in the Simpson matter , 1995, Nature Genetics.

[13]  Julia Mortera,et al.  Bounding the number of contributors to mixed DNA stains. , 2002, Forensic science international.

[14]  William C. Thompson,et al.  Beyond Bad Apples: Analyzing the Role of Forensic Science in Wrongful Convictions , 2009 .

[15]  Jae Won Lee.  Statistical DNA Forensics: Theory, Methods and Computation , 2013 .

[16]  David J. Balding,et al.  Bayesian Networks and Probabilistic Inference in Forensic Science , 2011 .

[17]  Simon Ford,et al.  Sequential Unmasking: A Means of Minimizing Observer Effects in Forensic DNA Interpretation , 2008, Journal of forensic sciences.

[18]  Colin Aitken,et al.  Analysis of sampling issues using Bayesian networks , 2007 .

[19]  Wing K. Fung,et al.  Interpreting forensic DNA mixtures: allowing for uncertainty in population substructure and dependence , 2000 .

[20]  J M Curran,et al.  Interpreting DNA mixtures in structured populations. , 1999, Journal of forensic sciences.

[21]  W. G. Hill,et al.  Genetic Data Analysis II . By Bruce S. Weir, Sunderland, Massachusetts. Sinauer Associates, Inc.445 pages. ISBN 0-87893-902-4. , 1996 .

[22]  J A Lambert,et al.  Taking account of peak areas when interpreting mixed DNA profiles. , 1998, Journal of forensic sciences.

[23]  I W Evett,et al.  A guide to interpreting single locus profiles of DNA mixtures in forensic cases. , 1991, Journal - Forensic Science Society.

[24]  J Buckleton,et al.  The calculation of DNA match probabilities in mixed race populations. , 2000, Science & justice : journal of the Forensic Science Society.

[25]  A. Dawid,et al.  Probabilistic expert systems for DNA mixture profiling. , 2003, Theoretical population biology.

[26]  I. Evett,et al.  Interpreting DNA Evidence: Statistical Genetics for Forensic Scientists , 1998 .

[27]  Wing K. Fung,et al.  Evaluating forensic DNA mixtures with contributors of different structured ethnic origins: a computer software , 2003, International Journal of Legal Medicine.

[28]  B. Weir.  Genetic Data Analysis II. , 1997 .