A contribution to the discussion of probabilities and human hair comparisons

The paper by Gaudette and Keeping on "An Attempt at Determining Probabilities in Human Scalp Hair Comparison" in the Journal of Forensic Sciences (Vol. 19, No. 3, July 1974, pp. 599-606) has provoked considerable controversy. This paper highlights two of the sources of the controversy and shows how the probability, 1/4500, quoted by Gaudette and Keeping should be treated with caution. The necessity of the use of a likelihood ratio statistic is described. It is suggested that the hair examination form resulting from the responses to the questionnaire recently distributed by the authors and also the discussions at Quantico (Proceedings of the International Symposium on Forensic Hair Comparisons, 25-27 June 1985, Quantico VA) should be used to facilitate the collection of the data which will be necessary to enable a likelihood ratio statistic to be estimated effectively.     

Estimation of the quantity of a drug in a consignment from measurements on a sample

A consignment of individual packages is thought to contain illegal material, such as drugs, in some or all of the packages. A sample from the consignment is inspected and the quantity of drugs in each package of the sample is measured. It is desired to estimate the total quantity of drugs in the consignment. Sampling variation is present in the original measurements and it is not sufficient just to adjust the sample mean pro rata. An analysis is described which takes account of the uncertainty concerning the proportion of the packages that contain drugs and provides a probabilistic summary of the quantity of drugs in the consignment. In particular, a probabilistic lower bound for the quantity of drugs in the consignment is given, which is dependent on the required standard of proof. This is in contrast to the approach based on confidence intervals which assumes that in the long run, the interval will contain the correct quantity the appropriate proportion of the time, but gives no measure of uncertainty associated with the particular consignment under consideration.     

Decision analysis in forensic science

Forensic scientists are routinely faced with the problems of making decisions under circumstances of uncertainty (i.e., to perform or not perform a test). A decision making model in forensic science is proposed, illustrated with an example from the field of forensic genetics. The approach incorporates available evidence and associated uncertainties with the assessment of utilities (or desirability of the consequences). The paper examines a general example for which identification will be made of the decision maker, the possible actions, the uncertain states of nature, the possible source of evidence and the kind of utility assessments required. It is argued that a formal approach can help to clarify the decision process and give a coherent means of combining elements to reach a decision.     

A Two‐Level Model for Evidence Evaluation in the Presence of Zeros*

Abstract: Likelihood ratios (LRs) provide a natural way of computing the value of evidence under competing propositions. We propose LR models for classification and comparison that extend the ideas of Aitken, Zadora, and Lucy and Aitken and Lucy to include consideration of zeros. Instead of substituting zeros by a small value, we view the presence of zeros as informative and model it using Bernoulli distributions. The proposed models are used for evaluation of forensic glass (comparison and classification problem) and paint data (comparison problem). Two hundred and sixty‐four glass samples were analyzed by scanning electron microscopy, coupled with an energy dispersive X‐ray spectrometer method and 36 acrylic topcoat paint samples by pyrolysis gas chromatography hyphened with mass spectrometer method. The proposed LR model gave very satisfactory results for the glass comparison problem and for most of the classification tasks for glass. Results of comparison of paints were also highly satisfactory, with only 3.0% false positive answers and 2.8% false negative answers.     

Information‐Theoretical Assessment of the Performance of Likelihood Ratio Computation Methods

Performance of likelihood ratio (LR) methods for evidence evaluation has been represented in the past using, for example, Tippett plots. We propose empirical cross‐entropy (ECE) plots as a metric of accuracy based on the statistical theory of proper scoring rules, interpretable as information given by the evidence according to information theory, which quantify calibration of LR values. We present results with a case example using a glass database from real casework, comparing performance with both Tippett and ECE plots. We conclude that ECE plots allow clearer comparisons of LR methods than previous metrics, allowing a theoretical criterion to determine whether a given method should be used for evidence evaluation or not, which is an improvement over Tippett plots. A set of recommendations for the use of the proposed methodology by practitioners is also given.     

The likelihood approach to compare populations: a study on DNA evidence and pitfalls of intuitions

The paper follows on from earlier work [Taroni F and Aitken CGG. Probabilistic reasoning in the law, Part 1: assessment of probabilities and explanation of the value of DNA evidence. Science & Justice 1998; 38: 165-177]. Different explanations of the value of DNA evidence were presented to students from two schools of forensic science and to members of fifteen laboratories all around the world. The responses were divided into two groups; those which came from a school or laboratory identified as Bayesian and those which came from a school or laboratory identified as non-Bayesian. The paper analyses these responses using a likelihood approach. This approach is more consistent with a Bayesian analysis than one based on a frequentist approach, as was reported by Taroni F and Aitken CGG. [Probabilistic reasoning in the law, Part 1: assessment of probabilities and explanation of the value of DNA evidence] in Science & Justice 1998.     

Competence in caring in legal practice

This article is concerned with the stories of female solicitors working in Queensland, Australia, and their account of ethics in practice. These narratives were sought and made in the context of our project investigating complaints received against practitioners in this jurisdiction. Our interviews with female lawyers were intended to provide more insight into complaints matters. Yet this discussion revealed broader insights into ethical questions about the nature of lawyer–client relationships and legal professionalism. This article considers these accounts by reflecting on the concept of ‘care’ and its fit with legal practice requirements today. In doing so, we consider the difficult gendered question of the relationship between caring and ‘women's’ practice. We do not assert a true woman's working style; we seek to contextualise our empirical research by tracing the complex effect of gender on lawyering roles. Finally, we suggest that the adoption of caring practice is a valid approach to lawyering and call for a caring approach to be re-valued in the legal professional context as an ethical proposition.