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

Joffe discussion of Leucari paper - 20 March 2006 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 Fundamental attributes Schum: (1)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: (1)value of information (2) conditional probability tables (3) likelihood ratio

Joffe discussion of Leucari paper - 20 March 2006 X3X3 X2X2 X4X4 Explaining away: “where knowledge of one being true lowers the probability of the other being true” X 3 : Sacco was involved in other crimes; X 2 : Sacco was involved in this crime; X 4 : Sacco intended to escape from the police when they arrested him

Joffe discussion of Leucari paper - 20 March 2006 Potential problems I there could be other reasons for X 4 (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 (X 3 ), this might increase not decrease the level of suspicion re this crime (X 2 ).

Joffe discussion of Leucari paper - 20 March 2006 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 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 A typology of graphical methods THOUGHT PROCESSES – generalisability requires justification of structure, links, etc BAYES NETS – “correct” subjective beliefs about objective/qu ve 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 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 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 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