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
Bayesian networks:
(1) value of information
(2) conditional probability tables
(3) likelihood ratio
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)”?
Joffe discussion of Leucari paper 20 March 2006
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
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
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 nondirectional – 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/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
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
Joffe discussion of Leucari paper 20 March 2006