Law, Probability and Risk - current issue

Seventh International Conference on Forensic Inference and Statistics, The University of Lausanne, Switzerland, August 2008

Aitken, C. G. G. — Thu, 12 Nov 2009 04:59:45 PST

A science of evidence: contributions from law and probability

Schum, D. A. — Thu, 12 Nov 2009 04:59:45 PST

Presented in this paper are arguments that studies of evidence in the fields of law and probability constitute a science of evidence when they are examined in light of five criteria for scientific activity. Three examples of studies from law and probability are presented to justify this argument. Examples from law include a method for classifying recurrent forms and combinations of evidence regardless of their substance or content, studies identifying credibility attributes for different forms of evidence and studies of complex argument construction in which new lines of inquiry and evidence are generated or discovered. Examples from probability include various views among probabilists about what the weight or force of evidence means and how it should be assessed, several examples of how important subtleties or complexities in evidential reasoning can be captured for study and analysis and one example of how alternative theories about the meaning of complex combinations of events can be tested empirically. All evidence has three major credentials: relevance, credibility and inferential force or weight. The law and probability studies cited in this paper concern all these credentials.

Forensics without uniqueness, conclusions without individualization: the new epistemology of forensic identification

Cole, S. A. — Thu, 12 Nov 2009 04:59:45 PST

Among the causes of the current sense that the forensic identification disciplines are ‘under siege’ are conceptual difficulties in these disciplines. Forensic identification disciplines either claim to achieve or strive to achieve conclusions of ‘individualization’, the reduction of the donor pool to a single source. They tend to support such claims by reference to the supposed ‘uniqueness’ of their objects of analysis. Both these notions remain extremely salient among practitioners and courts. And yet, a broad consensus in the forensic literature holds that individualization is unachievable and uniqueness is largely irrelevant to supporting claims of individualization. Focusing on latent print evidence, this article provides a clear articulation of the need to make a clean break from both individualization and uniqueness as forensic concepts. It argues that trace evidence disciplines can live without these concepts, and it explores what defensible conclusions might look like and how they might be supported.

Painting the target around the matching profile: the Texas sharpshooter fallacy in forensic DNA interpretation

Thompson, W. C. — Thu, 12 Nov 2009 04:59:45 PST

Forensic DNA analysts tend to underestimate the frequency of matching profiles (and overestimate likelihood ratios) by shifting the purported criteria for a ‘match’ or ‘inclusion’ after the profile of a suspect becomes known—a process analogous to the well-known Texas sharpshooter fallacy. Using examples from casework, informal and naturalistic experiments, and analysts’ own testimony, this article demonstrates how post hoc target shifting occurs and how it can distort the frequency and likelihood ratio statistics used to characterize DNA matches, making matches appear more probative than they actually are. It concludes by calling for broader adoption of more rigorous analytical procedures, such as sequential unmasking, that can reduce the sharpshooter fallacy by fixing the target before the shots are taken.

How accurate are the power calculations relied on by the SEC in its regulatory deliberations?

Bura, E., Gastwirth, J. L. — Thu, 12 Nov 2009 04:59:45 PST

In two related decisions in Chamber of Commerce of the United States of America v. Securities and Exchange Commission (SEC), the District of Columbia Federal Court of Appeals ruled that the SEC had not fully complied with some provisions of the Administrative Procedures Act when it required that the boards of investment companies managing mutual funds have at least 75% of their membership and the Chairman be independent directors. In preparation for renewed rule making, the Office of Economic Analysis of the SEC prepared a Power Study to respond to an industry-sponsored report claiming that the returns of funds with independent boards and chairs are not superior to funds with boards dominated by management. The Power Study concluded that the available studies on the effectiveness of independent board members do not have sufficient statistical power to detect a meaningful difference in the returns of the two types of funds. This paper demonstrates that the method used by the SEC in their power calculation is not correct, unless a very restrictive condition that rarely occurs in practice holds. When the appropriate power formulas are used, the expected power of studies of the same size as the ones examined by the SEC is actually lower than the corresponding results of the SEC. Thus, the results in the paper actually strengthen the argument that the SEC is advocating. The relevance of both the SEC and the industry studies to the main issue in the case is also questioned in the discussion.

Match likelihood ratio for uncertain genotypes

Perlin, M. W., Kadane, J. B., Cotton, R. W. — Thu, 12 Nov 2009 04:59:45 PST

Genetic data are not necessarily fully informative, leading to uncertainty in an inferred genotype. The posterior genotype probability distribution incorporates the identification information present in the data. To compare uncertain genotypes, we introduce here a match likelihood ratio (MLR), a simple generalization of the likelihood ratio standardly used to understand the import of genetic evidence in forensic applications. The MLR gives the relative probability of a match between questioned evidence and a suspect, with respect to a match between the evidence and a relevant population. Coancestry can be naturally incorporated. We present illustrative examples and provide a detailed analysis and comparison for a two-person DNA mixture. We describe MLR's computation efficiencies when making multiple genotype comparisons and show how MLR was used to explain evidence in court. As statistical computing of forensic DNA inferences becomes more commonplace, the MLR may help in quantifying match identification information.