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…