A Practical Guide for the Formulation of Propositions in the Bayesian Approach to DNA Evidence Interpretation in an Adversarial Environment

The interpretation of complex DNA profiles is facilitated by a Bayesian approach. This approach requires the development of a pair of propositions: one aligned to the prosecution case and one to the defense case. This note explores the issue of proposition setting in an adversarial environment by a series of examples. A set of guidelines generalize how to formulate propositions when there is a single person of interest and when there are multiple individuals of interest. Additional explanations cover how to handle multiple defense propositions, relatives, and the transition from subsource level to activity level propositions. The propositions depend on case information and the allegations of each of the parties. The prosecution proposition is usually known. The authors suggest that a sensible proposition is selected for the defense that is consistent with their stance, if available, and consistent with a realistic defense if their position is not known.     

What is the magnitude of the subpopulation effect?

The effect of population subdivision on estimated match probabilities has been raised [Nature 339 (1989) 501; Am. J. Hum. Genet. 48 (1991) 819; Science 254 (1991) 1921]. Previous work [J. Forensic Sci. 39 (1994) 319; J. Forensic Sci. 39 (1994) 988; Am. J. Hum. Genet. 55 (1994) 533] has compared product rule estimates from differing databases and found that the "subpopulation" error may be of the order of a factor of 10. This approach compares an estimate with an estimate. This paper uses simulation to extend these studies by allowing a comparison to a 'true match probability' and supports the conclusion that subpopulation effects are mild. In addition the performance of recommendations 4.1 and 4.2 of NRC II [National Research Council and C.O.D.F. Science, The Evaluation of Forensic DNA Evidence, National Academy Press, Washington, DC, 1996].     

Logical implications of applying the principles of population genetics to the interpretation of DNA profiling evidence

There have been several efforts to codify the approach to interpreting DNA evidence [National Research Council, The Evaluation of Forensic DNA Evidence, National Academy Press, Washington, DC, 1996; I.W. Evett, B.S. Weir, Interpreting DNA Evidence: Statistical Genetics for Forensic Scientists Sinauer, Sunderland, MA, 1998]. Despite these efforts there are still aspects of ad hoc decision making in modern DNA interpretation. This article discusses some of the remaining areas of concern in this respect. Because of the immense discriminating power of DNA evidence it is unlikely that these concerns would contribute to a miscarriage of justice. They are more likely to lead to lengthy and wasteful debate in court, and to potential appeals. We advocate a previously developed approach to DNA evidence [Sci. Justice 39 (4) (1999) 257; B.S. Weir, in: D.J. Balding, C. Cannings, M. Bishop (Eds.), Handbook of Statistical Genetics, Wiley Series in Probability and Statistics, Wiley, New York, ISBN: 0-471-86094-8, 2001; J. R. Stat. Soc. A 158 (1) (1995) 21] that would give a more solid logical foundation and hopefully lead to sounder and less debatable testimony.     

DNA commission of the International Society of Forensic Genetics: Recommendations on the interpretation of mixtures

The DNA commission of the International Society of Forensic Genetics (ISFG) was convened at the 21st congress of the International Society for Forensic Genetics held between 13 and 17 September in the Azores, Portugal. The purpose of the group was to agree on guidelines to encourage best practice that can be universally applied to assist with mixture interpretation. In addition the commission was tasked to provide guidance on low copy number (LCN) reporting. Our discussions have highlighted a significant need for continuing education and research into this area. We have attempted to present a consensus from experts but to be practical we do not claim to have conveyed a clear vision in every respect in this difficult subject. For this reason, we propose to allow a period of time for feedback and reflection by the scientific community. Then the DNA commission will meet again to consider further recommendations.     

The effect of linkage on the calculation of DNA match probabilities for siblings and half siblings

The loci in many forensic multiplexes are often selected to avoid linked loci. However as the multiplices used increase in the number of loci represented instances are occurring of loci that are loosely linked. As yet little attention has been paid to the likely consequence of this. We begin the process of developing formulae to give the match probability at two linked loci for full and half siblings. The methodology proceeds from the previously published joint IBD states for two linked loci [B.K. Suarez, J. Rice, T. Reich, The generalized sib pair IBD distribution: its use in the detection of linkage, Ann. Hum. Genet. Lond. 42 (1978) 87; J.K. Haseman, R.C. Elston, The investigation of linkage between a quantitative trait and a marker locus, Behav. Genet. 2 (1972) 3-19]. Our formulation has the drawback of assuming linkage equilibrium. This assumption may be tenable as a first order approximation. We hope to stimulate work on developing better treatments.     

Autosomal microsatellite allele frequencies for a nationwide dataset from the Australian Caucasian sub-population

DNA profiling results presented in court must be accompanied by a statistical estimate of its evidential weight. In calculating such statistics, allele frequencies from the tested loci are required. This paper reports allele frequencies and the results of population genetic testing of datasets of autosomal microsatellite profiles from Australian Caucasian donors. In contrast to previous practice in Australia these data have been collated at the national level rather than at the State and Territory level. We consider that this national dataset could be used in forensic DNA casework throughout Australia as previously recommended by Ayres et al. [K.L. Ayres, J. Chaseling, D.J. Balding, Implications for DNA identification arising from an analysis of Australian forensic databases, Forensic Sci. Int. 129 (2002) 90-98].     

The impact of Māori cultural values on forensic science practice in New Zealand

Forensic science aims to serve society by advancing justice. It is accepted that some actions taken by the state in the interests of advancing justice, such as postmortem examinations, may impinge on values held by members of groups within society. Such actions have the potential to cause cultural offense. It is important that forensic scientists are aware of these issues and that as a profession we should take actions, where possible, to reduce any potential offense and consequently reduce unnecessary distress. This paper examines the impact of these issues on forensic practice in New Zealand, and, in particular, in relation to the cultural values of Māori, the indigenous people of New Zealand. Interviews and workshops were used to identify forensic practices involving a risk of cultural offense. Particular issues were identified in regard to crime scene attendance and examination, postmortem attendance and sample storage, disposal, and return. This paper describes the response developed by the Institute of Environmental Science and Research Limited (ESR) to address these issues, including a cultural awareness training package and reference brochure.     

Applications and extensions of subpopulation theory: a caseworkers guide

This paper extends the calculation of conditional probabilities from those given by Balding and Nichols to casework situations where a series of possible DNA types are possible. Such situations may occur when a sample is identified containing a mixture of DNA from two or more people or where extra information can be determined about the subpopulation under consideration by analysis of additional samples. Using this approach, the error in the estimated likelihood ratios is expected to reduce as the number of additional individuals typed from the subpopulation increases.