Forensic science evidence in question

How should forensic scientists and other expert witnesses present their evidence in court? What kinds and quality of data can experts properly draw on in formulating their conclusions? In an important recent decision in R. v T1 the Court of Appeal revisited these perennial questions, with the complicating twist that the evidence in question incorporated quantified probabilities, not all of which were based on statistical data. Recalling the sceptical tenor of previous judgments addressing the role of probability in the evaluation of scientific evidence,2 the Court of Appeal in R. v T condemned the expert’s methodology and served notice that it should not be repeated in future, a ruling which rapidly reverberated around the forensic science community causing consternation, and even dismay, amongst many seasoned practitioners.3 At such moments of perceived crisis it is essential to retain a sense of perspective. There is, in fact, much to welcome in the Court of Appeal’s judgment in R. v T, starting with the court’s commendable determination to subject the quality of expert evidence adduced in criminal litigation to searching scrutiny. English courts have not consistently risen to this challenge, sometimes accepting rather too easily the validity of questionable scientific techniques.4 However, the Court of Appeal’s reasoning in R. v T is not always easy to follow, and there are certain passages in the judgment which, taken out of context, might even appear to confirm forensic scientists’ worst fears. This article offers a constructive reading of R. v T, emphasising its positive features whilst rejecting interpretations which threaten, despite the Court of Appeal’s best intentions, to diminish the integrity of scientific evidence adduced in English criminal trials and distort its probative value

Immediate access arteriovenous grafts versus tunnelled central venous catheters: study protocol for a randomised controlled trial

Background Autologous arteriovenous fistulae (AVF) are the optimal form of vascular access for haemodialysis. AVFs typically require 6 to 8 weeks to “mature” from the time of surgery before they can be cannulated. Patients with end-stage renal disease needing urgent vascular access therefore traditionally require insertion of a tunnelled central venous catheter (TCVC). TCVCs are associated with high infection rates and central venous stenosis. Early cannulation synthetic arteriovenous grafts (ecAVG) provide a novel alternative to TCVCs, permitting rapid access to the bloodstream and immediate needling for haemodialysis. Published rates of infection in small series are low. The aim of this study is to compare whether TCVC ± AVF or ecAVG ± AVF provide a better strategy for managing patients requiring immediate vascular access for haemodialysis. Methods/design This is a prospective randomised controlled trial comparing the strategy of TCVC ± AVF to ecAVG ± AVF. Patients requiring urgent vascular access will receive a study information sheet and written consent will be obtained. Patients will be randomised to receive either: (i) TCVC (and native AVF if this is anatomically possible) or (ii) ecAVG (± AVF). 118 patients will be recruited. The primary outcome is systemic bacteraemia at 6 months. Secondary outcomes include culture-proven bacteraemia rates at 1 year and 2 years; primary and secondary patency rates at 3, 6, 12 and 24 months; stenoses; re-intervention rates; re-admission rate; mortality and quality of life. Additionally, treatment delays, impact on service provision and cost-effectiveness will be evaluated. Discussion This is the first randomised controlled trial comparing TCVC to ecAVG for patients requiring urgent vascular access for haemodialysis. The complications of TCVC are considered an unfortunate necessity in patients requiring urgent haemodialysis who do not have autologous vascular access. If this study demonstrates that ecAVGs provide a safe and practical alternative to TCVC, this could instigate a paradigm shift in nephrology thinking and access planning.</p

Evidence evaluation for discrete data

Methods for the evaluation of evidence in the form of measurements by means of the likelihood ratio are becoming more widespread. There is a paucity of methods for the evaluation of evidence in the form of counts by means of the likelihood ratio. The outline of an empirical method based on relative frequencies that takes account of similarity and rarity is described. It is compared with two methods based on an assumption of independence of counts and one assuming dependence between adjacent Bernoulli variables. Examples of their performance are illustrated in the context of a problem in forensic phonetics. There is discussion of the problems particular to the evaluation of evidence for discrete data, with suggestions for further work

Statistical Interpretation of Evidence: Bayesian Analysis

Probability theory provides the general framework within which assignments of probabilities of past, present, and future events are coherently modified in the light of observed events or, more generally, new information. Forensic scientists, as an illustrative example, routinely face tasks of reasoning under uncertainty when they seek to assist members of the judiciary in evaluating or interpreting the meaning of items of scientific evidence. As a consequence of the laws of probability theory and related concepts, Bayes’ theorem is the key rule according to which to conduct such reasoning in order to comply with the requirement of rationality. This quantification, though, does not represent the end of the matter as the forensic scientist may also be confronted with questions of how to make a rational choice amongst alternative courses of action. This article presents the role of Bayes’ theorem, and its extension to decision analysis, in categorical and continuous data analysis in forensic science applications. It emphasizes the importance of propositional hierarchies, the role of background information, the interpretation of probability as personal degrees of belief and the personal quantification of the consequences of decisions. The discussion also includes a sketch of some common pitfalls of intuition associated with probabilistic reasoning in legal contexts

A generalised Bayes' factor formula for evidence evaluation under activity level propositions: variations around a fibres scenario

Generalised Bayes' factors and associated Bayesian networks are developed for the transfer of extrinsic evidence at the activity level, developments that extend previous work on activity level evaluation. A strategy for the assessment of extrinsic evidence is developed in stages with progressive increases in complexity. The final development is illustrated with an example involving fibres from clothing. This provides a list of factors involved in the consideration of a transfer case with activity level propositions and their roles in the determination of evidential value

Transformations for compositional data with zeros with an application to forensic evidence evaluation

In forensic science likelihood ratios provide a natural way of computing the value of evidence under competing propositions such as "the compared samples have originated from the same object" (prosecution) and "the compared samples have originated from different objects" (defence). We use a two-level multivariate likelihood ratio model for comparison of forensic glass evidence in the form of elemental composition data under three data transformations: the logratio transformation, a complementary log-log type transformation and a hyperspherical transformation. The performances of the three transformations in the evaluation of evidence are assessed in simulation experiments through use of the proportions of false negatives and false positives

The evaluation of evidence for exponentially distributed data

At present, likelihood ratios for two-level models are determined with the use of a normal kernel estimation procedure when the between-group distribution is thought to be non-normal. An extension is described here for a two-level model in which the between-group distribution is very positively skewed and an exponential distribution may be thought to represent a good model. The theoretical likelihood ratio is derived. A likelihood ratio based on a biweight kernel with an adaptation at the boundary is developed. The performance of this kernel is compared alongside those of normal kernels and normal and exponential parametric models. A comparison of performance is made for simulated data where results may be compared with those of theory, using the theoretical model, as the true parameter values for the models are known. There is also a comparison for forensic data, using the concentration of aluminium in glass as an exemplar. Performance is assessed by determining the numbers of occasions on which the likelihood ratios for sets of fragments from the same group are supportive of the proposition that they are from different groups and the numbers of occasions on which the likelihood ratios for sets of fragments from different group are supportive of the proposition that they are from the same group

Assessing the probative value of DNA evidence: guidance for judges, lawyers, forensic scientists and expert witnesses

Practical guidance for judges, lawyers, forensic scientists and expert witnesses on the logical analysis of DNA profiles, and their probative value in criminal proceedings. Explains technical aspects of DNA profiling and how this information should be presented and interpreted in criminal trials