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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.