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Recently, Lund and Iyer (L&I) raised an argument regarding the use of likelihood ratios in court. In our view, their argument is based on a lack of understanding of the paradigm. L&I argue that the decision maker should not accept the expert's likelihood ratio without further consideration. This is agreed by all parties. In normal practice, there is often considerable and proper exploration in court of the basis for any probabilistic statement. We conclude that L&I argue against a practice that does not exist and which no one advocates. Further we conclude that the most informative summary of evidential weight is the likelihood ratio. We state that this is the summary that should be presented to a court in every scientific assessment of evidential weight with supporting information about how it was constructed and on what it was based., (Copyright © 2018 Elsevier B.V. All rights reserved.)

A recent report by the US President's Council of Advisors on Science and Technology (PCAST), (2016) has made a number of recommendations for the future development of forensic science. Whereas we all agree that there is much need for change, we find that the PCAST report recommendations are founded on serious misunderstandings. We explain the traditional forensic paradigms of match and identification and the more recent foundation of the logical approach to evidence evaluation. This forms the groundwork for exposing many sources of confusion in the PCAST report. We explain how the notion of treating the scientist as a black box and the assignment of evidential weight through error rates is overly restrictive and misconceived. Our own view sees inferential logic, the development of calibrated knowledge and understanding of scientists as the core of the advance of the profession., (Copyright © 2017 Elsevier B.V. All rights reserved.)

The interpretation of complex DNA profiles is facilitated by a Bayesian approach. This approach requires the development of a pair of propositions: one aligned to the prosecution case and one to the defense case. This note explores the issue of proposition setting in an adversarial environment by a series of examples. A set of guidelines generalize how to formulate propositions when there is a single person of interest and when there are multiple individuals of interest. Additional explanations cover how to handle multiple defense propositions, relatives, and the transition from subsource level to activity level propositions. The propositions depend on case information and the allegations of each of the parties. The prosecution proposition is usually known. The authors suggest that a sensible proposition is selected for the defense that is consistent with their stance, if available, and consistent with a realistic defense if their position is not known., (© 2015 American Academy of Forensic Sciences.)

The value of forensic results crucially depends on the propositions and the information under which they are evaluated. For example, if a full single DNA profile for a contemporary marker system matching the profile of Mr A is assessed, given the propositions that the DNA came from Mr A and given it came from an unknown person, the strength of evidence can be overwhelming (e.g., in the order of a billion). In contrast, if we assess the same result given that the DNA came from Mr A and given it came from his twin brother (i.e., a person with the same DNA profile), the strength of evidence will be 1, and therefore neutral, unhelpful and irrelevant(1) to the case at hand. While this understanding is probably uncontroversial and obvious to most, if not all practitioners dealing with DNA evidence, the practical precept of not specifying an alternative source with the same characteristics as the one considered under the first proposition may be much less clear in other circumstances. During discussions with colleagues and trainees, cases have come to our attention where forensic scientists have difficulty with the formulation of propositions. It is particularly common to observe that results (e.g., observations) are included in the propositions, whereas-as argued throughout this note-they should not be. A typical example could be a case where a shoe-mark with a logo and the general pattern characteristics of a Nike Air Jordan shoe is found at the scene of a crime. A Nike Air Jordan shoe is then seized at Mr A's house and control prints of this shoe compared to the mark. The results (e.g., a trace with this general pattern and acquired characteristics corresponding to the sole of Mr A's shoe) are then evaluated given the propositions 'The mark was left by Mr A's Nike Air Jordan shoe-sole' and 'The mark was left by an unknown Nike Air Jordan shoe'. As a consequence, the footwear examiner will not evaluate part of the observations (i.e., the mark presents the general pattern of a Nike Air Jordan) whereas they can be highly informative. Such examples can be found in all forensic disciplines. In this article, we present a few such examples and discuss aspects that will help forensic scientists with the formulation of propositions. In particular, we emphasise on the usefulness of notation to distinguish results that forensic scientists should evaluate from case information that the Court will evaluate., (Copyright © 2015 The Chartered Society of Forensic Sciences. Published by Elsevier Ireland Ltd. All rights reserved.)

There has been a recent push from many jurisdictions for the standardisation of forensic DNA interpretation methods. Current research is moving from threshold-based interpretation strategies towards continuous interpretation strategies. However laboratory uptake of software employing probabilistic models is slow. Some of this reluctance could be due to the perceived intimidating calculations to replicate the software answers and the lack of formal internal validation requirements for interpretation software. In this paper we describe a set of experiments which may be used to internally validate in part probabilistic interpretation software. These experiments included both single source and mixed profiles calculated with and without dropout and drop-in and studies to determine the reproducibility of the software with replicate analyses. We do this by way of example using three software packages: STRmix™, LRmix, and Lab Retriever. We outline and demonstrate the profile examples where the expected answer may be calculated and provide all calculations., (Copyright © 2014 Elsevier Ireland Ltd. All rights reserved.)

When a forensic scientist presents the results of a comparison between a DNA profile from a questioned sample with that of a defendant the interpretation will be based on the premise of a given number of contributors. It is quite common practice for defence counsel to ask how consideration of a greater number of putative contributors to the profile would affect the interpretation. This note discusses the response to such a request., (Copyright © 2014 Forensic Science Society. Published by Elsevier Ireland Ltd. All rights reserved.)

This paper reports on the purpose, design, methodology and target audience of E-learning courses in forensic interpretation offered by the authors since 2010, including practical experiences made throughout the implementation period of this project. This initiative was motivated by the fact that reporting results of forensic examinations in a logically correct and scientifically rigorous way is a daily challenge for any forensic practitioner. Indeed, interpretation of raw data and communication of findings in both written and oral statements are topics where knowledge and applied skills are needed. Although most forensic scientists hold educational records in traditional sciences, only few actually followed full courses that focussed on interpretation issues. Such courses should include foundational principles and methodology - including elements of forensic statistics - for the evaluation of forensic data in a way that is tailored to meet the needs of the criminal justice system. In order to help bridge this gap, the authors' initiative seeks to offer educational opportunities that allow practitioners to acquire knowledge and competence in the current approaches to the evaluation and interpretation of forensic findings. These cover, among other aspects, probabilistic reasoning (including Bayesian networks and other methods of forensic statistics, tools and software), case pre-assessment, skills in the oral and written communication of uncertainty, and the development of independence and self-confidence to solve practical inference problems. E-learning was chosen as a general format because it helps to form a trans-institutional online-community of practitioners from varying forensic disciplines and workfield experience such as reporting officers, (chief) scientists, forensic coordinators, but also lawyers who all can interact directly from their personal workplaces without consideration of distances, travel expenses or time schedules. In the authors' experience, the proposed learning initiative supports participants in developing their expertise and skills in forensic interpretation, but also offers an opportunity for the associated institutions and the forensic community to reinforce the development of a harmonized view with regard to interpretation across forensic disciplines, laboratories and judicial systems., (Copyright © 2012 Elsevier Ireland Ltd. All rights reserved.)

The authors have proposed a quantitative method for assessing weight of evidence in the case where a fingermark from a crime scene is compared with a set of control prints from the ten fingers of a suspect. The approach is based on the notion of calculating a Likelihood Ratio (LR) that addresses a pair of propositions relating to the individual who left the crime mark. The current method considers only information extracted from minutiae, such as location, direction and type. It does not consider other information usually taken into account by fingerprint examiners, such as the general pattern of the ridge flow on the mark and the control prints. In this paper, we propose an improvement to our model that allows a fingerprint examiner to take advantage of pattern information when assessing the evidential weight to be assigned to a fingerprint comparison. We present an extension of the formal analysis proposed earlier and we illustrate our approach with an example., (Copyright © 2011 Elsevier Ireland Ltd. All rights reserved.)

This is a discussion of a number of issues that arise from the recent judgment in R v T [1]. Although the judgment concerned with footwear evidence, more general remarks have implications for all disciplines within forensic science. Our concern is that the judgment will be interpreted as being in opposition to the principles of logical interpretation of evidence. We reiterate those principles and then discuss several extracts from the judgment that may be potentially harmful to the future of forensic science. A position statement with regard to evidence evaluation, signed by many forensic scientists, statisticians and lawyers, has appeared in this journal [2] and the present paper expands on the points made in that statement., (Copyright © 2011 Forensic Science Society. Published by Elsevier Ireland Ltd. All rights reserved.)

The authors have published elsewhere a quantitative method for assessing weight of evidence in the case where a finger mark from a crime scene is compared with a control print taken from a single finger of a suspect. The approach is based on the notion of calculating a likelihood ratio (LR) that addresses a pair of propositions relating to the single finger that was the origin of the crime mark. In practice, things are rather different because the crime mark will not just be compared with a single finger from a suspect but with a set of prints from all of his/her fingers; likewise, when the mark is compared with a database, this will consist of ten print records from random individuals. It is clear that "finger propositions" are not realistic in this situation and we show how our approach may be generalised to address a pair of propositions that relate to the person that made the crime mark. It often is the case that information is present at the crime scene that enables some inference to be drawn relating to which of the offender's ten fingers left a particular mark of interest. This kind of inference may profitably be drawn into the formal analysis. We illustrate our approach with an example., (Copyright © 2010 Elsevier Ireland Ltd. All rights reserved.)

We consider the problem of drawing inferences within a legal framework when a person is a suspect for two separate offences. Although we are primarily concerned with scientific evidence the issue inevitably arises as to how that evidence interacts with other, non-scientific evidence. We show that, in this particular context, the evidence can be conveniently classified into three categories that concern, respectively: the first crime only; the second crime only; and evidence that relates to similarities between the two crimes. Two case examples are considered and we consider DNA, fibres and eyewitness evidence. These are viewed from the perspective of a prosecutor who has to decide whether or not to charge a suspect with one or both crimes. Graphical sensivity analyses are presented which have features that are not intuitively obvious.

The questions that are asked of forensic scientists during the course of a criminal investigation, and during subsequent court proceedings, are of varied form. This paper attempts to place these questions into broad generic types and explores the difference in the inferential process that a scientist may employ when forming opinions that help answer these questions. From this model, a working definition of different roles and attributes for forensic scientists is described which may offer greater clarity for both practitioners and users of forensic science.

Previous papers in Science & Justice have described the work of the Case Assessment and Interpretation (CAI) project that has been running for several years within the Forensic Science Service (FSS). The principles of the CAI model, which have developed through casework, are the foundation of a balanced, robust and logical approach to interpretation. The question arises frequently as to what is the most appropriate database that should be available to assist in assigning a value to a given probability. In this paper we present a set of guidelines in the form of flowcharts and explore them within the context of a range of case examples.

The dramatic increase in the sensitivity of DNA profiling systems that has occurred over recent years has led to the need to address a wider range of interpretational problems in forensic science. The issues surrounding questions of the kind "whose DNA is this?" have been the subject of considerable controversy but now it is clear that the emphasis is shifting to questions of the kind "how did this DNA get here?" Such issues are discussed in this paper and new insights are provided by two particular recent developments. First, the notion of the "hierarchy of propositions" that has arisen from a project called Case Assessment and Interpretation (CAI) that has been running in the British Forensic Science Service (FSS). Second, a technique for drawing inferences in the face of many interacting considerations, known as "Bayesian networks"--or "Bayes' nets" for short--that has been the subject of an earlier paper in this journal (1). The discussion is carried out by means of case studies, based on actual cases. It is clear that, whereas the inference in relation to the source of the DNA in a crime sample might be overwhelmingly strong, the inference in relation to the propositions that a jury must consider relating to the identity of the actual offender may be much more tentative.

The use of earmarks as evidence in criminal trials appears to be expanding, but there is something of a dearth of peer-reviewed scientific publications to support the pursuit. This paper is a critical review of the current literature in which we emphasize the weaknesses of the present state of knowledge. Some research directions are proposed to gather statistical knowledge of the within-source and between-source variability of earmarks and earprints. Its ultimate goal is to be able to assess likelihood ratios in relation to this type of evidence.

We report the use of DNA profiles from six STR loci for inferring the ethnic origin of a crime stain and discuss how such inference may be used as intelligence information to reduce the expected number of interviews to resolve a case. To enable this work, databases have been created for five British ethnic groups (Caucasians, Afro-Caribbeans, Indian sub-continentals, Southeast Asians and Middle Easterners) that together comprise 99.7% of the UK population.

A new ten-locus STR (short tandem repeat) profiling system was recently introduced into casework by the Forensic Science Service (FSS) and statistical analyses are described here based on data collected using this new system for the three major racial groups of the UK: Caucasian. Afro-Caribbean and Asian (of Indo-Pakistani descent). Allele distributions are compared and the FSS position with regard to routine significance testing of DNA frequency databases is discussed. An investigation of match probability calculations is carried out and the consequent analyses are shown to provide support for proposed changes in how the FSS reports DNA results when very small match probabilities are involved.

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.

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.

When evaluating DNA evidence, the necessary calculations are often carried out using databases drawn from broad populations; for example, the Forensic Science Service (FSS) maintains genetic databases for the 3 major racial groups of England and Wales--Caucasian, Afro-Caribbean and Asian (from the Indian subcontinent). The resulting figures may be challenged in court on the premise that they are not based on data from the population of most relevance in the particular case under consideration. One important factor might be the location of the crime. Since the recent establishment of a National DNA Intelligence Database, data have been made available from a wide range of geographical regions in England and Wales. This paper gives details of analyses conducted to measure the differentiation between white caucasian populations from these regions and from other areas of the UK and abroad using a Bayesian approach.

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.

The interpretation of mixtures of DNA in the forensic context presents particular challenges. The only logical means available for dealing with them is through Bayesian inference, which leads to the formulation, in most cases, of a likelihood ratio which weighs the evidence in favour of two competing hypotheses. However, situations can arise in which additional hypotheses are proposed and the authors discuss one such situation--that where the number of contributors to the mixture is in dispute. A way of dealing with the problem is presented.

A previous paper in this journal has described the conventional statistical analysis of three databases (Caucasian, Afro-Caribbean and Asians from the Indian subcontinent) where individuals are typed at six short tandem repeat (STR) loci. This paper presents a Bayesian analysis of the same data and the approach is centred on the concept of estimating coancestry coefficients from mixed databases. Posterior distributions for the three databases are presented and the discussed and the consequences of implementing bootstrap estimation procedures are also shown.

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 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.

Before the introduction of a four-locus multiplex short-tandem-repeat (STR) system into casework, an extensive series of tests were carried out to determine robust procedures for assessing the evidential value of a match between crime and suspect samples. Twelve databases were analyzed from the three main ethnic groups encountered in casework in the United Kingdom: Caucasians, Afro-Caribbeans, and Asians from the Indian subcontinent. Independence tests resulted in a number of significant results, and the impact that these might have on forensic casework was investigated. It is demonstrated that previously published methods provide a simple procedure for correcting allele frequencies--and that this leads to conservative casework estimates of evidential value.

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.

Over 5,700 three-probe VNTR DNA profiles collected by several United Kingdom (UK) laboratories have been compared to examine the probability of randomly matching 2 samples from different individuals. In over 16 million comparisons, using a matching rule corresponding to the matching guideline employed by the UK Forensic Science Service, no profiles were found to match at the 3 loci D1S7 (MS1), D7S21(MS31) and D12S11 (MS43a). The frequency of occurrence of a set of Caucasian profiles have been estimated with 6 reference databases. The results show that there were greater differences in the frequency estimates when using a database of Afro-Caribbean or Asian profiles, rather than a different Caucasian database. The results further demonstrate the power and robustness of the VNTR DNA profiling technique for forensic casework.

The debate about the statistics of DNA profiling in forensic science casework has been carried out mainly from the perspective which is generally known as "match/binning." This approach has an initial appeal because of its apparent conceptual simplicity. However, the simplicity is illusory because it encourages misconceptions which obscure the essential forensic issues. This is exemplified in a recent report of the National Research Council, which places great emphasis on the need for conservative estimation of relative frequencies while missing the point that the power of RFLP technology cannot be realized if the matching stage is inefficient. Our approach to the problem is a one-stage rather than a two-stage process, by means of one function--the likelihood ratio--which determines the evidential strength. This paper describes experiments which have been carried out to assess the power of the method in forensic science and compares it with match/binning methodology. Tests for gauging the effects of between-probe dependence are included, with the results complementing those of Risch and Devlin.

A small experiment is described which uses databases of Afro-Caribbeans and Caucasians to establish the quality of information that might be given to an investigator about the ethnic group of the donor of an unknown crime stain. The results show that the technique can provide qualified indications, but categoric conclusions are not, in general, feasible. An appraisal is given of the technique within the operational context.

This paper describes a simplified approach to calculating the Bayesian likelihood ratio for the case where two DNA single locus profiles are to be compared. It explains how the calculation allows for band shift and also for the variation in precision with molecular weight. A simple basic theory section explains the principles of the analysis but more detailed explanations are given in an advanced theory section. Experiments on substantial data collections are described which demonstrate the robustness of the method.

We describe an experiment to investigate the potential effectiveness of an international DNA database for linking unsolved crimes using the data from a collaborative exercise among members of the European DNA Profiling Group. We show that effective retrieval of unknown samples will be feasible even from large databases provided that the conditions that were adopted for the collaborative exercise are maintained.

The widespread application of DNA profiling has brought to some prominence the issue of close relatives sharing body fluid types. In a particular crime case it may be suggested that a close relative of the suspect was the person responsible. This paper explains how an assessment may be made in the context of single locus profiling and where the suggestion is that a full sibling is involved. A formula is established for the likelihood ratio which is then worked out in detail for the particular case where the suspect profile has two distinct bands. Generalization to other types of case is briefly explained and there is a short discussion of the impact of the evidence in the context of a trial. For all parties, justice will best be done if the scientist has ample notice, before the trial, of relationship issues such as this one.

The history of DNA profiling in the Home Office Forensic Science Service began with the introduction of multilocus probes into casework in 1986. The use of single-locus probes was introduced in 1990, supported by databases of three ethnic groups; interpretation is backed up using a Bayesian approach. Databases were compiled using an image analysis computing system. Quality control systems are described, detailing requirements before a sample can be included in the database.

We discuss three issues relating to the evaluation of DNA single locus probe comparisons in forensic casework: band shift, database size, and population stratification. Illustrations are from experiments carried out in the Forensic Science Service as part of our preparations to implement the technique.