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CATEGORIES:Isaac Newton Institute Seminar Series
SUMMARY:Prior Probability and the Presumption of Innocence
- Christian Dahlman (Lund University)
DTSTART;TZID=Europe/London:20160929T133000
DTEND;TZID=Europe/London:20160929T141500
UID:TALK67706AThttp://talks.cam.ac.uk
URL:http://talks.cam.ac.uk/talk/index/67706
DESCRIPTION:My talk will address a problem of fundamental impo
rtance for the Bayesian approach to evidence asse
ssment in criminal cases. How shall a court\, oper
ating under the presumption of innocence\, determ
ine the prior probability that the defendant is g
uilty\, before the evidence has been presented? I
will examine some ways to approach this problem\,
and review different solutions. The consideratio
ns that determine the prior probability can be epi
stemic or normative. If they are purely epistemic
\, the fact that the defendant has been selected
for prosecution must be considered as evidence for
guilt\, and this violates the presumption of inn
ocence (Dawid 1993\, 12). The prior probability m
ust therefore be determined completely or partly o
n normative grounds. It has been suggested by Den
nis Lindley and others that the prior probability
shall be determined as 1/N\, where N is the numbe
r of people who could have committed the act that
the defendant is accused of (Lindley 1977\, 218\;
Dawid 1993\, 11\; Bender & Nack 1995\, 236)\, bu
t there are several objections to this solution. A
s Leonard Jaffee has pointed out\, the prior prob
ability will not be equal in all criminal trials\
, as N will vary from case to case (Jaffee 1988\,
978). This is problematic since the doctrine of f
air trial requires that defendants are treated eq
ually. Furthermore\, the court will not have suffi
cient knowledge about all possible scenarios to d
etermine N with the robustness required by the st
andard of proof (Dahlman\, Wahlberg & Sarwar 2015\
, 19). My suggestion for the problem is that the
prior probability should be determined completely
on normative grounds\, by assigning a standardize
d number to N\, for example N = 100. If the numbe
r of people who could have committed the crime is
always presumed to be 100\, the probability that
the defendant is guilty before the evidence has b
een presented will be 1% in all trials. According
to this solution\, the prior probability is an in
stitutional fact (Searle 1995\, 104).
LOCATION:Seminar Room 1\, Newton Institute
CONTACT:INI IT
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