SummaryThe paper provides a review of current issues relating to the use of DNA profiling in forensic science. A short historical section gives the main statistical milestones that occurred during a rapid development of DNA technology and operational uses. Greater detail is then provided for interpretation issues involving STR DNA profiles, including:– methods that take account of population substructure in DNA calculations;– parallel work carried out by the US National Research Council;– the move away from multiple independence testing in favour of experiments that demonstrate the robustness of casework procedures;– the questionable practice of source attribution ‘with reasonable scientific certainty’;– the effect on the interpretation of profiles obtained under increasingly sensitive techniques, the LCN technique in particular;– the use of DNA profiles as an intelligence tool;– the interpretation of DNA mixtures.Experience of presenting DNA evidence within UK courts is also discussed. The paper then summarises a generic interpretation framework based on the concept of likelihood ratio within a hierarchy of propositions. Finally the use of Bayesian networks to interpret DNA evidence is reviewed.

The Forensic Science Service (FSS) has devoted appreciable effort to developing the application of the principles of evidence interpretation. Much of the work has been reported in previous papers in this journal, in particular those that develop a model for Case Assessment and Interpretation (CAI). The principles of interpretation are restated and the implications for structure and content of statements are described.

DNA profiling has brought to the courts a new way of looking at forensic science evidence. The weight of evidence, where there is a match between the profiles of a defendant and a crime sample, is presented int he form of a match probability. In all other areas of forensic science, it is long accepted practice for the scientist to give an opinion of the form "in my opinion, x and y have the same source" but recent judgments have established that this is not to be permitted when x and y are DNA profiles. Yet DNA profiling is better understood from a statistical standpoint than any other forensic techniques, including fingerprints and, as profiling techniques become more powerful, so the match probabilities can be expected to become smaller. This paper discusses issues relating to how such probabilities should be presented at court.

Interpretation of the weight of scientific evidence depends upon the framing of at least two competing propositions to weigh against each other. It is the stage of framing propositions that is the most difficult aspect of evidence interpretation. The logical structure for case assessment and interpretation has been described by the authors in three previous papers [Cook R, et al. A model for case assessment and interpretation. Science & Justice 1998; 38: 151-156. Cook R, et al. A hierarchy of propositions: deciding which level to address in casework. Science & Justice 1998; 38: 231-239. Cook R, et al. Case pre-assessment and review in a two-way transfer case. Science & Justice 1999; 39: 103-111]. This paper considers the framing of propositions in greater detail, in particular the intermediate stage of exploring less formal explanations. All of the discussion is based on experiences encountered in workshops with caseworking forensic scientists.

In previous papers in this journal [Cook R, Evett IW, Jackson G, Jones PJ and Lambert JA. A model for case assessment and interpretation. Science & Justice 1998; 38: 151-156. Cook R, Evett Mi, Jackson G, Jones PJ and Lambert JA. A hierarchy of propositions: deciding which level to address in casework. Science & Justice 1998; 38: 231-239], the authors have described a model for case assessment and interpretation. This paper continues the discussion by studying the pre-assessment stage in more detail, using a hypothetical fibres case in which there may be a two-way transfer. It is demonstrated that a simple multiplication of likelihood ratios for the two directions of transfer is not valid because of dependencies in the numerator. The solution to this problem is described by means of a is made. Next the paper shows how the pre-assessment is made. Next the paper shows how the pre-assessment can be updated when a staged approach is taken: the results of the examination of one of the garments are used to inform the decision about whether the second garment should be examined.

The authors describe a new approach to decision-making in an operational forensic science organization based on a model, embodying the principles of Bayesian inference, which has been developed through workshops run within the Forensic Science Service for forensic science practitioners. Issues which arise from the idea of pre-assessment of cases are explored by means of a 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.

This paper describes an attempt to formalise the interpretation of footwear marks. First, the definitions of identification and individualisation which were given by Kirk are discussed and formalised, then a Bayesian analysis is presented in which the assumptions made are clarified. The analysis is broken down into components which reflect different interpretative issues. Application of a formal expression for the likelihood ratio is then illustrated by means of examples from casework in New Zealand and the United Kingdom.

Data have been collected from 602 Caucasians, 190 Afro-Caribbeans and 257 Asians of Indo/Pakistani descent who have been profiled using a new six locus short tandem repeat (STR) multiplex. The data have been analysed by conventional significance testing methods: the exact test, homozygosity, and conventional goodness of fit to Hardy-Weinberg proportions. Frequency tables are given and the expected performance in British forensic casework is discussed.

Blood samples from approximately 200 Scottish Caucasian individuals were typed at conventional loci (PGM, Gc and EAP) and also with a four locus STR multiplex. Tests of the data are described which demonstrate that the assumptions of between locus independence are robust for use in forensic casework.

Data from nearly 2500 British Caucasians, profiled using an STR quadruplex, have been analysed. The data came from several laboratories and represent samples from different geographical distributions. Analysis of the combined files shows that previous reports of failed independence tests were the results of sampling effects. A further convincing proof is given of the robustness of the statistical methods used to estimate evidential value in casework. Comparisons between different samples show that regional effects between Scotland and the South of England have no importance from the forensic viewpoint.

Data for four STR loci have been collected from 400 samples taken from complainers and suspects encountered in casework at the Strathclyde Police Forensic Science Laboratory (SPFSL). This paper describes statistical testing which demonstrates that its use will provide operationally robust procedures. Comparisons made with data collected from other British samples confirmed no practical differences between the different frequency distributions. This work provides further confirmation of the reliability of the so-called ‘product rule’ in estimating the frequency of multilocus genotypes in British forensic casework.

This is a continuation of discussions in other papers in this journal by the authors. A Bayesian perspective is used to explore issues of glass evidence interpretation. Data from two surveys of glass on clothing are used and there is a discussion of transfer/persistence probabilities, by considering the sensitivity of the likelihood ratio, leading to some remarks on knowledge elicitation. An analysis is suggested for dealing with post hoc explanations for the presence of glass on a suspect's clothing.

The statistical significance of sampling is one consideration amongst others for a forensic scientist when deciding how many recovered items of the same generic type to compare with a control sample. These statistical considerations can be pursued using the hypergeometric distribution. The probabilities of selecting samples of particular numbers of matching and non-matching items from populations of items comprising different numbers of matching and non-matching items have been computed using the hypergeometric distribution for situations commonly arising in the casework examination of glass. The implications of the results have been considered.