Fibres evidence, probabilistic evaluation and collaborative test.
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Forensic Medicine↗
Biomedical subjects
C G Aitken
Publications and source records attributed to C G Aitken.
DNA evidence, probabilistic evaluation and collaborative tests.
Forensic scientists working in 12 state or private laboratories participated in collaborative tests to improve the reliability of the presentation of DNA data at trial. These tests were motivated in response to the growing criticism of the power of DNA evidence. The experts' conclusions in the tests are presented and discussed in the context of the Bayesian approach to interpretation. The use of a Bayesian approach and subjective probabilities in trace evaluation permits, in an easy and intuitive manner, the integration into the decision procedure of any revision of the measure of uncertainty in the light of new information. Such an integration is especially useful with forensic evidence. Furthermore, we believe that this probabilistic model is a useful tool (a) to assist scientists in the assessment of the value of scientific evidence, (b) to help jurists in the interpretation of judicial facts and (c) to clarify the respective roles of scientists and of members of the court. Respondents to the survey were reluctant to apply this methodology in the assessment of DNA evidence.
Alleles↗
Sampling--how big a sample?
It is thought that, in a consignment of discrete units, a certain proportion of the units contain illegal material. A sample of the consignment is to be inspected. Various methods for the determination of the sample size are compared. The consignment will be considered as a random sample from some super-population of units, a certain proportion of which contain drugs. For large consignments, a probability distribution, known as the beta distribution, for the proportion of the consignment which contains illegal material is obtained. This distribution is based on prior beliefs about the proportion. Under certain specific conditions the beta distribution gives the same numerical results as an approach based on the binomial distribution. The binomial distribution provides a probability for the number of units in a sample which contain illegal material, conditional on knowing the proportion of the consignment which contains illegal material. This is in contrast to the beta distribution which provides probabilities for the proportion of a consignment which contains illegal material, conditional on knowing the number of units in the sample which contain illegal material. The interpretation when the beta distribution is used is much more intuitively satisfactory. It is also much more flexible in its ability to cater for prior beliefs which may vary given the different circumstances of different crimes. For small consignments, a distribution, known as the beta-binomial distribution, for the number of units in the consignment which are found to contain illegal material, is obtained, based on prior beliefs about the number of units in the consignment which are thought to contain illegal material. As with the beta and binomial distributions for large samples, it is shown that, in certain specific conditions, the beta-binomial and hypergeometric distributions give the same numerical results. However, the beta-binomial distribution, as with the beta distribution, has a more intuitively satisfactory interpretation and greater flexibility. The beta and the beta-binomial distributions provide methods for the determination of the minimum sample size to be taken from a consignment in order to satisfy a certain criterion. The criterion requires the specification of a proportion and a probability.
Bayes Theorem↗
Evaluation of enzyme immunoassays in the diagnosis of camel (Camelus dromedarius) trypanosomiasis: a preliminary investigation.
Three enzyme immunoassays were used for the serodiagnosis of Trypanosoma evansi in camels in the Sudan in order to evaluate their ability to discriminate between infected and non-infected animals. Two assays were used for the detection of trypanosomal antibodies, one using specific anti-camel IgG conjugate and another using a non-specific Protein A conjugate. The third assay detected the presence of trypanosomal antigens using anti-T. evansi antibodies in a double antibody sandwich assay. Inspection of the frequency distribution of assay results suggested that the ELISA for circulating trypanosomal antibodies using specific antisera and the ELISA for circulating antigens can distinguish between non-infected camels and infected camels exhibiting patent infections or not. The ELISA using Protein A conjugate to bind non-specifically to camel immunoglobulin did not appear to discriminate between infected and non-infected animals.
Animals↗
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.
Forensic Medicine↗
The value of microscopic features in the examination of human head hairs: statistical analysis of questionnaire returns.
There is an apparent diversity of opinion with respect to the probative value of hair in the United Kingdom and in North America. A questionnaire was devised and widely circulated in an attempt to discover the reasons behind this diversity and to seek the views of a broad range of hair examiners. This paper reports on the statistical analyses of the answers to the closed form questions. The majority of replies to most questions favored the number of categories given in the questionnaire. The general impression is that U.K. scientists tend to want fewer categories for classification of microscopic features than the North Americans. The largest divisions of opinion concerned pigment distribution and density and the medulla, where North American scientists want more categories for classification and the U.K. scientists fewer. The implications of these results in the choice and description of features to be examined in human hairs is considered in the second paper concerned with this questionnaire.
Forensic Medicine↗
The value of microscopic features in the examination of human head hairs: analysis of comments contained in questionnaire returns.
Comments received in response to a questionnaire seeking opinion about hair examination have been considered. The questionnaire was in two parts: the first was concerned with the description of microscopic features and the second with the use of numerical features and data sheets in hair examinations. The same format is used to present a synopsis of the many individual comments and suggestions contained in questionnaire returns. It is argued that a hair examination form can contribute to more effective hair examinations although not replacing direct side-by-side comparisons. A hair examination form is presented with recommendations for its use.
Forensic Medicine↗
Association between tail injuries and docking in dogs.
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Animals↗
Statistical modelling in specific case analysis.
In spite of the problems associated with the development of a statistical approach in specific case analysis, the potential of two statistical methods, logistic regression and Bayesian belief networks, has been investigated and found encouraging in the context of a database relating to child murders with a sexual connotation. Continual collaboration between statisticians and detectives would be needed in the collection of the data, the choice and construction of the models, and the interpretation of the results.
Adult↗
Probabilistic reasoning in the law. Part 1: Assessment of probabilities and explanation of the value of DNA evidence.
Problems associated with the interpretation of DNA evidence are investigated with reference to measures of uncertainty associated with the presentation of such evidence in an adversarial context. The use of probabilistic arguments associated with expert scientific testimony and cases where DNA evidence has been presented in court are studied. A series of problems were developed for, and given to, forensic medicine students, forensic science students and practitioners (advocates and forensic scientists) in Scotland to investigate their understanding of uncertainty. In the first part, four different methods of presentation of the value of DNA evidence were given to the participants and their responses are described. In the second part, different explanations of the value of DNA evidence as given by experts in court were given to the participants and their responses are described. The authors suggest improvements for the presentation of scientific evidence and for the education of future lawyers and forensic scientists.
DNA↗
Probabilistic reasoning in the law. Part 2: Assessment of probabilities and explanation of the value of trace evidence other than DNA.
Problems associated with the interpretation of scientific evidence (fibres and glass) in forensic science are investigated with reference to measures of uncertainty associated with the presentation of such evidence in an adversarial context. The investigation includes the use of probabilistic arguments associated with expert scientific testimony in the courts. Cases where scientific evidence has been presented in court are studied. A series of problems were developed for, and given to, forensic medicine students' forensic science students and practitioners (advocates and forensic scientists) in Scotland to investigate their understanding of uncertainty.
Bayes Theorem↗
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.
Bayes Theorem↗