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A quadruplex system for determining the genetic profile of an individual at four short tandem repeat (STR) loci has recently been introduced into forensic casework by the Forensic Science Service (FSS), primarily for the purposes of forensic identification. Data have been collected under this system from the three racial groups of most relevance in casework in the UK: Caucasian. Afro-Caribbean and Asian (from the Indian subcontinent). These data are utilized in calculations to quantify the evidential strength of a DNA match between suspect and crime scene sample, say, through the evaluation of a likelihood ratio (LR). Previous papers (1,2) have studied the databases via classical statistical methods. However, we focus on a Bayesian approach (3) to validation of the data for LR evaluation in two main cases: when individuals being compared are either (i) completely unrelated, or (ii) members of the same racial group subpopulation. Empirical studies are conducted to establish the robustness of proposed models and obtain efficient and adequate approximations to the LR calculations. This involves the use of statistical simulation methods to determine the suitability of the product rule and Bayesian inference for coancestry coefficients in the absence of subpopulation data.

The forensic scientist often faces the task of interpreting patterns of evidence which involve many variables. Combining different items of evidence within a complex framework of circumstances requires logical powers of reasoning and this can be assisted by formal methods. We discuss one such method which, as has already been pointed out by Aitken and Gammerman (1), offers considerable potential for creating probabilistic expert systems to assist in evidence interpretation. In particular, we show how the method, which is based on a directed acyclic graph, enables dependencies between different aspects of the evidence to be considered. The discussion is based on an imaginary case example.

This paper establishes a logical framework for taking account of peak areas when interpreting mixed DNA STR profiles. The principles apply wherever such data are available but they are illustrated here by means of data which have been collected from made up mixtures of known concentrations analyzed at short tandem repeat loci. The data have led to some modeling assumptions which are used for numerical examples. In actual casework the proportions of the various components will not be known and there is a discussion of whether they should be allowed for by integrating over a prior distribution. This is a conceptual paper, rather than a prescription for casework, and the scope for further work is outlined.

A recent case is described where the evidence of bloodstaining on a knife suggested that it was a mixture from the two victims. Interpretation of the evidence in this problem necessitated the formulation of several sets of multiple hypotheses which were analyzed by means of a tree diagram. The problem was then greatly simplified to one of comparing the two alternative hypotheses of most interest. It was found that results were robust to variation in the expert's judgment regarding the possibility that a mixture of blood was present on the knife.

Verbal Conventions for Handwriting Opinions