Statistical basis for positive identification in forensic anthropology

Abstract

Forensic scientists are often expected to present the likelihood of DNA identifications in US courts based on comparative population data, yet forensic anthropologists tend not to quantify the strength of an osteological identification. Because forensic anthropologists are trained first and foremost as physical anthropologists, they emphasize estimation problems at the expense of evidentiary problems, but this approach must be reexamined. In this paper, the statistical bases for presenting osteological and dental evidence are outlined, using a forensic case as a motivating example. A brief overview of Bayesian statistics is provided, and methods to calculate likelihood ratios for five aspects of the biological profile are demonstrated. This paper emphasizes the definition of appropriate reference samples and of the "population at large," and points out the conceptual differences between them. Several databases are introduced for both reference information and to characterize the "population at large," and new data are compiled to calculate the frequency of specific characters, such as age or fractures, within the "population at large." Despite small individual likelihood ratios for age, sex, and stature in the case example, the power of this approach is that, assuming each likelihood ratio is independent, the product rule can be applied. In this particular example, it is over three million times more likely to obtain the observed osteological and dental data if the identification is correct than if the identification is incorrect. This likelihood ratio is a convincing statistic that can support the forensic anthropologist's opinion on personal identity in court.

Bayes in biological anthropology

“…Here we expand on victim identification, making the simplifying assumption that there is no commingling of remains so that we are solely attempting to make identifications. Rather than working directly with posterior probabilities as in the previous section, we will work with likelihood ratios and prior odds as this is much more common in the disaster victim and personal identification literature (Goodwin et al, 1999;Adams, 2003b;Brenner and Weir, 2003;Alonso et al, 2005;Christensen, 2005;Lin et al, 2006;Steadman et al, 2006;Prinz et al, 2007;Kaye, 2009;Budowle et al, 2011;Butler, 2011;Hartman et al, 2011;Abraham et al, 2012;Montelius and Lindblom, 2012;Jackson and Black, 2013). As mentioned above, likelihood ratios from DNA are typically reported as the inverse of the population frequency for the matched (between ante-mortem and post-mortem) genotype, although this is only possible when a "direct reference" ante-mortem sample is available (so that the numerator is equal to 1.0).…”

“…As an example of this more conservative approach, Steadman et al (2006) found that a particular pattern of dental pathology observed in a forensic case was never observed in a database sample of 29,152 individuals. They used a match probability of 1/29,153 in analyzing their case, on the presumption that had one additional case been sampled it would have yielded a match.…”

“…In a critique of Steadman et al (2006), Ferreira and Andrade (2009) confuse the issue of a prior on an identification with a prior for the denominator calculation in a likelihood ratio. Steadman et al (2006) gave the likelihood ratio as:…”

Amelia Earhart and the Nikumaroro Bones: A 1941 Analysis versus Modern Quantitative Techniques

“…In jury trials, experts are often advised to testify only to the likelihood ratio developed from the biological evidence. The jury then supplies its own prior odds based on the entire context (e.g., Steadman et al 2006).…”

Some Achievements and Challenges of Dental Anthropology

“…[23][24][25][26][27]. Considering that tooth structure is not remodeled during life, they offer a large amount of information for understanding the biological and social dynamics of past populations [3,14,28,29] and assist in the identification of missing persons through forensic anthropology procedures [30][31][32]. Modern Dental Anthropology is the result of systematic efforts carried out by research teams for decadesin order to strengthen the scientific nature of the discipline and tried to explain the enormous biological diversity of human populations.The vast amount of information generated byscholars such as A.Hrdlička, A. Dahlberg, S. Garn, R. Potter, G. Townsend, E.Harris, J. Irish, K. and T.Hanihara, J. Kieser, A. Goodman, K. Alt, S.Hillson,H.…”