Attacking Non-Bayesian Reasoning William Thompson UC Irvine February 15, 2014
Feb 23, 2016
Attacking Non-Bayesian Reasoning
William Thompson UC Irvine
February 15, 2014
Do Jurors Give More of Less Weight Than They Should to Forensic Science?
• Importance to the law of evidence• Claims of Underweighting– Finkelstein & Fairley (1970) , citing Edwards (1968);
Slovic & Lichtenstein (1971)• Claims of Overweighting– Tribe (1971)
• Jury Simulation Studies– E.g., Schklar & Diamond (1999); Nance & Morris,
(2002; 2005)
Figure 1: Log Scale for Estimating Chances Defendant is Guilty
___Certain to be guilty___About 9,999,999 chances in 10 million that he is guilty___About 999,999 chances in 1 million that he is guilty___About 99,999 chances in 100,000 that he is guilty___About 9,999 chances in 10,000 that he is guilty___About 999 chances in 1,000 that he is guilty___About 99 chances in 100 that he is guilty___About 9 chances in 10 that he is guilty___One chance in 2 (fifty-fifty chance) that he is guilty___About 1 chance in 10 that he is guilty___About 1 chance in 100 that he is guilty___About 1 chance in 1,000 that he is guilty___About 1 chance in 10,000 that he is guilty___About 1 chance in 100,000 that he is guilty___About 1 chance in 1 million that he is guilty___About 1 chance in 10 million that he is guilty___Impossible that he is guilty
What are the chances that an innocent man in a case like this would, just by coincidence, happen to match the DNA left by another man at the crime scene?– 1 in 10– 1 in 100– 1 in 1000– 1 in 10,000– 1 in 100,000– 1 in 1 million– 1 in 1 billion– Zero chances--impossible
Guilty (G) Not Guilty(NG)
Framed (F) FUP FUP
Not Framed
(NF)
1-FUP 1-FUP
Match (M) No Match(NM)
Match Reported
(R)
1 FRP
Not Reported
0 1-FRP
Framed (F) Not Framed (NF)
G NG G NG
Match (M) 1 1 1 RMPNo Match
(NM)0 0 0 1-RMP
Figure 5: Bayesian Network Model for Evaluating the Probative Value of the Forensic Evidence Based on Individual Perceptions of the RMP, FRP and FUP
A DNA Match Between Perpetrator and Suspect: Is the suspect guilty?
The DNA Expert Says:
The prosecutor says:
But the defense lawyer says:
So what are the chances he’s guilty?
How many of the matching people are plausible suspects?
We don’t really know…