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Formal tools for handling evidence – Dr Valentina Leucari

Formal tools for handling evidence – Dr Valentina Leucari. Discussion by Dr Mike Joffe. Strengths of the paper. comparing and contrasting different types of graphical method is useful – Bayesian networks and Wigmore charts here

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Formal tools for handling evidence – Dr Valentina Leucari

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  1. Formal tools for handling evidence – Dr Valentina Leucari Discussion by Dr Mike Joffe Joffe discussion of Leucari paper - 20 March 2006

  2. Strengths of the paper • comparing and contrasting different types of graphical method is useful – Bayesian networks and Wigmore charts here • decomposing large models into smaller component ones is very useful • the idea of having modular components – what the paper calls “recurrent structures of evidence” could be extremely valuable Joffe discussion of Leucari paper - 20 March 2006

  3. Schum: relevance (2) credibility (3) strength p18: “credibility (2) is represented by likelihood ratios (3)” p33: “no conditional independence (2) in Wigmore charts, but still notion of relevance (1)” does this mean that “strength” (3) corresponds to “value of information (1)”? Bayesian networks: value of information (2) conditional probability tables (3) likelihood ratio Fundamental attributes Joffe discussion of Leucari paper - 20 March 2006

  4. Explaining away: “where knowledge of one being true lowers the probability of the other being true” X3 X2 X4 X3: Sacco was involved in other crimes; X2: Sacco was involved in this crime; X4: Sacco intended to escape from the police when they arrested him Joffe discussion of Leucari paper - 20 March 2006

  5. Potential problems I • there could be other reasons for X4 (apart from the policeman making this up), e.g. a generalisation “everyone fears the police”, or “immigrants are treated as suspect”; or “Sacco’s political activity caused him to fear arrest”, or “Sacco was paranoid” • if Sacco were involved in other crimes (X3), this might increase not decrease the level of suspicion re this crime (X2). Joffe discussion of Leucari paper - 20 March 2006

  6. Potential problems II • also, it’s not clear from the charts what the process is – e.g. the chart for “explaining away” (fig 3.11) looks similar to that for the “filter fragment” (fig 3.9) and to that for Event/Competence/Sensation (fig 3.7) etc • and how would this be interpreted in the case of a causal model? –interpretation is clear when it’s a case of a belief making another less likely (p6: “A model can be causal”) Joffe discussion of Leucari paper - 20 March 2006

  7. Different languages I • conditional probabilities (or more generally, joint distributions) are in themselves non-directional – a direction (arrow-head) is only present if imposed (e.g. blocks in graphical models) • are we dealing with objective causal relations here, or subjective belief systems? or both? – ‘Bayesian networks are a “process model” … intended to capture a complex process by which some series of events could have been generated’ (Schum 2005 – see p33) Joffe discussion of Leucari paper - 20 March 2006

  8. A typology of graphical methods • THOUGHT PROCESSES – generalisability requires justification of structure, links, etc • BAYES NETS – “correct” subjective beliefs about objective/quve causal relationships • CAUSAL RELATIONSHIPS – arrows that represent causation in the world; combines a priori specification and empirical testing • STATISTICAL ASSOCIATIONS – represent joint distributions only; links non-directional Joffe discussion of Leucari paper - 20 March 2006

  9. Different languages II • different languages (here verbal reasoning and the laws of probability): the questions are • (a) when and how to use each of them • (b) how to interrelate them – to manage boundary compatibility Joffe discussion of Leucari paper - 20 March 2006

  10. Grouping of items of evidence • the suggestion in section 6.2 (page 37) that items of evidence could be grouped into Witness evidence, Physical evidence and Consciousness of guilt evidence is problematic – it would be better to group items according to sub-stories, e.g. whether the suspect was present, who hit the guard, etc • more broadly, I would welcome the idea of alternative stories being made more explicit Joffe discussion of Leucari paper - 20 March 2006

  11. How special is Law? • in this programme, we have focused a great deal on legal examples – what special considerations does this introduce? • we should consider using journalism as a focus: it is more rooted in “commonsense + science”, undistorted by arbitrary rules of evidence (although in practice distorted by commercialism), and not constrained to the guilt or not of particular people • responsibility not culpability Joffe discussion of Leucari paper - 20 March 2006

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