1 Fundamentals of Probability and Statistical Evidence in

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PRACTITIONER GUIDE NO 1, Fundamentals of Probability and Statistical Evidence. in Criminal Proceedings, Guidance for Judges Lawyers Forensic Scientists and Expert Witnesses. Colin Aitken Professor of Forensic Statistics University of Edinburgh. Paul Roberts Professor of Criminal Jurisprudence University of Nottingham. Graham Jackson Professor of Forensic Science Abertay University. Prepared under the auspices of the, Royal Statistical Society s Working Group on Statistics and the Law. Chairman Colin Aitken,0 Introduction 3, 1 Probability and statistics in forensic contexts 13. 2 Basic concepts of probabilistic inference and evidence 27. 3 Interpreting probabilistic evidence anticipating traps for the 53. 4 Summary and checklist 81,Appendices,A Glossary 88.
B Technical elucidation and illustrations 102, C Select case law precedents and further illustrations 113. D Select bibliography 118, Introduction to Communicating and Interpreting Statistical Evidence. in the Administration of Criminal Justice,0 1 Context Motivation and Objectives. Statistical evidence and probabilistic reasoning today play an important and expanding. role in criminal investigations prosecutions and trials not least in relation to forensic. scientific evidence including DNA produced by expert witnesses It is vital that. everybody involved in criminal adjudication is able to comprehend and deal with. probability and statistics appropriately There is a long history and ample recent. experience of misunderstandings relating to statistical information and probabilities which. have contributed towards serious miscarriages of justice. 0 2 English and Scottish criminal adjudication is strongly wedded to the principle of lay fact. finding by juries and magistrates employing their ordinary common sense reasoning. Notwithstanding the unquestionable merits of lay involvement in criminal trials it cannot. be assumed that jurors or lay magistrates will have been equipped by their general. education to cope with the forensic demands of statistics or probabilistic reasoning This. predictable deficit underscores the responsibilities of judges and lawyers within the. broader framework of adversarial litigation to present statistical evidence and. probabilities to fact finders in as clear and comprehensible a fashion as possible Yet legal. professionals grasp of statistics and probability may in fact be little better than the. average juror s, Perhaps somewhat more surprisingly even forensic scientists and expert witnesses whose. evidence is typically the immediate source of statistics and probabilities presented in. court may also lack familiarity with relevant terminology concepts and methods Expert. witnesses must satisfy the threshold legal test of competency before being allowed to. testify or submit an expert report in legal proceedings 1 However it does not follow from. the fact that the witness is a properly qualified expert in say fingerprinting or ballistics or. paediatric medicine that the witness also has expert or even rudimentary knowledge of. R v Atkins 2009 EWCA Crim 1876 R v Stockwell 1993 97 Cr App R 260 CA R v Silverlock. 1894 2 QB 766 CCR, statistics and probability Indeed some of the most notorious recent miscarriages of justice.
involving statistical evidence have exposed errors by experts. There is in short no group of professionals working today in the criminal courts that can. afford to be complacent about its members competence in statistical method and. probabilistic reasoning, 0 3 Well informed observers have for many decades been arguing the case for making basic. training in probability and statistics an integral component of legal education e g Kaye. 1984 But little tangible progress has been made It is sometimes claimed that lawyers. and the public at large fear anything connected with probability statistics or mathematics. in general but irrational fears are plainly no excuse for ignorance in matters of such great. practical importance More likely busy practitioners lack the time and opportunities to fill. in persistent gaps in their professional training Others may be unaware of their lack of. knowledge or believe that they understand but do so only imperfectly a little learning is. a dang rous thing 2, 0 4 If a broad programme of education for lawyers and other forensic practitioners is needed. in what should this consist and how should it be delivered It would surely be misguided. and a wasted effort to attempt to turn every lawyer judge and expert witness let alone. every juror into a professor of statistics Rather the objective should be to equip forensic. practitioners to become responsible producers and discerning consumers of statistics and. confident exponents of elementary probabilistic reasoning It is a question of each. participant in criminal proceedings being able to grasp at least enough to perform their. respective allotted roles effectively in the interests of justice. For the few legal cases demanding advanced statistical expertise appropriately qualified. statisticians can be instructed as expert witnesses in the normal way For the rest lawyers. need to understand enough to be able to question the use made of statistics or probabilities. and to probe the strengths and expose any weaknesses in the evidence presented to the. court judges need to understand enough to direct jurors clearly and effectively on the. statistical or probabilistic aspects of the case and expert witnesses need to understand. Alexander Pope An Essay on Criticism 1711, enough to be able to satisfy themselves that the content and quality of their evidence is. commensurate with their professional status and no less importantly with an expert. witness s duties to the court and to justice 3, 0 5 There are doubtless many ways in which these pressing educational needs might be met. and the range of possibilities is by no means mutually exclusive Of course design and. regulation of professional education are primarily matters to be determined by the relevant. professional bodies However in specialist matters requiring expertise beyond the. traditional legal curriculum it would seem sensible for authoritative practitioner guidance. to form a central plank of any proposed educational package This would ideally be. developed in conjunction with if not directly under the auspices of the relevant. professional bodies and education providers, The US Federal Judicial Center s Reference Manual on Scientific Evidence 2nd edn 2000.
provides a valuable and instructive template Written with the needs of a legal primarily. judicial audience in mind it covers a range of related topics including data collection. data presentation base rates comparisons inference association and causation multiple. regression survey research epidemiology and DNA evidence There is currently no. remotely comparable UK publication specifically addressing statistical evidence and. probabilistic reasoning in criminal proceedings in England and Wales Scotland and. Northern Ireland, 0 6 In association with the Royal Statistical Society RSS and with the support of the. Nuffield Foundation we aim to fill this apparent gap in UK forensic practitioner guidance. This is the first of four planned Practitioner Guides on aspects of statistical evidence and. probabilistic reasoning intended to assist judges lawyers forensic scientists and other. expert witnesses in coping with the demands of modern criminal litigation The Guides are. being written by a multidisciplinary team comprising a statistician Aitken an academic. lawyer Roberts and two forensic scientists Jackson and Puch Solis They are produced. under the auspices of the RSS s Working Group on Statistics and the Law whose. membership includes representatives from the judiciary the English Bar the Scottish. R v B T 2006 2 Cr App R 3 2006 EWCA Crim 417 176 And see CrimPR 2010 Rule. 33 2 Expert s duty to the court reproduced in Appendix B below. Faculty of Advocates the Crown Prosecution Service the National Police Improvement. Agency NPIA and the Forensic Science Service as well as academic lawyers. statisticians and forensic scientists, 0 7 Users Guide to this Guide Some Caveats and Disclaimers. Guide No 1 is designed as a general introduction to the role of probability and statistics in. criminal proceedings a kind of vade mecum for the perplexed forensic traveller or. possibly Everything you ever wanted to know about probability in criminal litigation but. were too afraid to ask It explains basic terminology and concepts illustrates various. forensic applications of probability and draws attention to common reasoning errors. traps for the unwary A further three Guides will be produced over the next three years. Building on the foundations laid by Guide No 1 they will address the following more. discrete topics in greater detail 2 DNA profiling evidence 3 networks for structuring. evidence and 4 case assessment and interpretation Each of these topics is of major. importance in its own right Their deeper exploration will also serve to elucidate and. exemplify the general themes concepts and issues in the communication and. interpretation of statistical evidence and probabilistic reasoning in the administration of. criminal justice which are introduced in the following pages. 0 8 This Guide develops a logical narrative in which each section builds on those which. precede it starting with basic issues of terminology and concepts and then guiding the. reader through a range of more challenging topics The Guide could be read from start to. finish as a reasonably comprehensive primer on statistics and probabilistic reasoning in. criminal proceedings Perhaps some readers will adopt this approach However we. recognise that many busy practitioners will have neither the time nor the desire to plough. through the next eighty odd pages in their entirety So the Guide is also intended to serve. as a sequence of self standing introductions to particular topics issues or problems which. the reader can dip in and out of as time and necessity direct Together with the four. appendices attached to this Guide we hope that this modular format will meet the. practical needs of judges lawyers and forensic scientists for a handy work of reference. that can be consulted possibly repeatedly whenever particular probability related issues. arise during the course of their work, 0 9 We should flag up at the outset certain challenges which beset the production of this kind. of Guide not least because it is likely that we have failed to overcome them entirely. satisfactorily, First we have attempted to address multiple professional audiences Insofar as there is a. core of knowledge skills and resources pertaining to statistical evidence and probabilistic. reasoning which is equally relevant for trial judges lawyers and forensic scientists and. other expert witnesses involved in criminal proceedings it is entirely appropriate and. convenient to pitch the discussion at this generic level The successful integration of. statistics and probabilistic reasoning into the administration of criminal justice is likely to. be facilitated if participants in the process are better able to understand other professional. groups perspectives assumptions concerns and objectives For example lawyers might. be able to improve the way they instruct experts and lead their evidence in court by. gaining insight into forensic scientists thinking about probability and statistics whilst. forensic scientists for their part may become more proficient as expert witnesses by. gaining a better appreciation of lawyers understandings and expectations of expert. evidence in particular regarding the salience and implications of its probabilistic. We recognise nonetheless that certain parts of the following discussion may be of greater. interest and practical utility to some criminal justice professionals than to others This is. another reason why readers might prefer to treat the following exposition and its. appendices more like a work of reference than a monograph Our hope is that judges. lawyers and forensic scientists will be able to extrapolate from the common core of. mathematical precepts and their forensic applications and adapt this generic information to. the particular demands of their own professional role in criminal proceedings For. example we hope to have supplied useful information that might inform the way in which. a trial judge might assess the admissibility of expert evidence incorporating a probabilistic. component or direct a jury in relation to statistical evidence but we have stopped well. short of presuming to specify formal criteria of legal admissibility or to formulate concrete. guidance that trial judges might repeat to juries We have neither the competence nor the. authority to made detailed recommendations on the law and practice of criminal. 0 10 The following exposition is also generic in a second sense directly related to the preceding. observations We hope that this Guide will be widely used in all of the United Kingdom s. legal jurisdictions It goes without saying that the laws of probability unlike the laws of. the land are valid irrespective of geography It would be artificial and sometimes. misleading when describing criminal litigation to avoid any reference whatsoever to legal. precepts and doctrines and we have not hesitated to mention legal rules where the context. Statistical Evidence in Criminal Proceedings Guidance for Judges Lawyers Forensic Scientists and Expert Witnesses Colin Aitken Paul Roberts Graham Jackson 1 PRACTITIONER GUIDE NO 1 Fundamentals of Probability and Statistical Evidence in Criminal Proceedings Guidance for Judges Lawyers Forensic Scientists and Expert Witnesses By Colin Aitken Professor of Forensic Statistics University

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