1. Attacking Non-Bayesian Reasoning William Thompson UC Irvine February 15, 2014

2. 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)

3. 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

4. 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

5. Guilty (G)Not Guilty (NG) Framed (F)FUP Not Framed (NF) 1-FUP Match (M)No Match (NM) Match Reported (R) 1FRP Not Reported 01-FRP Framed (F)Not Framed (NF) GNGG Match (M)111RMP No Match (NM) 0001-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

9. A DNA Match Between Perpetrator and Suspect: Is the suspect guilty?

10. The DNA Expert Says:

11. The prosecutor says:

12. But the defense lawyer says:

13. So what are the chances he’s guilty?

15. How many of the matching people are plausible suspects?

16. We don’t really know…