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Attacking DNA that Implicates Your Client William C. Thompson, J.D., Ph.D. Dept. of Criminology, Law & Society University of California, Irvine.

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Presentation on theme: "Attacking DNA that Implicates Your Client William C. Thompson, J.D., Ph.D. Dept. of Criminology, Law & Society University of California, Irvine."— Presentation transcript:

1 Attacking DNA that Implicates Your Client William C. Thompson, J.D., Ph.D. Dept. of Criminology, Law & Society University of California, Irvine

2 Attacking DNA Evidence Gross errors in handling, processing, labeling of samples Mistaken assumptions about sample source Someone else’s DNA is also present Inadvertent transfer of DNA Biased interpretation of test results Exaggerated or misleading statistics

3 What you see in the lab report: Which suspect is a possible source of the blood? Table of Alleles D3S135 8 VWAFGAAm el D8S117 9 D21S11D18S 51 D5S818D13S31 7 D7S820 Blood Stain 15, 1615, 1525, 26XY12,1327,3013,1410,119,1210,12 Suspect 116, 1815, 1621, 24XY12,1427,2813,1711,128,118,12 Suspect 215, 1518, 1819, 23XY13,1529, ,99,10 Suspect 315, 1615, 1525, 26XY12,1327,3013,1410,119,1210,12 Suspect 416, 1616, 1719, 24XY1430,3013,169,1110,119,10

4 What’s Behind the Lab Report

5 Are there unreported ambiguities?

6 Saliva Sample: D5S818 D13S317 D7S820 Defendant: Allelic Dropout or the Wrong Man?

7 Figure 4: Presence of more than two alleles at a locus indicates a mixture. Presence of more than two alleles at a locus indicates a mixture

8 The progressively smaller peak heights in this sample from left to right are indicative of degradation.

9 Technical Problems May Complicate Interpretation

10 Vaginal Swab—Male Fraction (showing defendant’s profile) Profile of Second Man (who was “excluded” by lab)

11 Vaginal Swab—Defense Reanalysis (showing evidence of second profile) Profile of Second Man (who was “excluded” by lab)

12 Inadvertent Transfer of DNA Primary transfer -- from individual to an object or another person –R. van Oorschot & M. Jones, DNA fingerprints from fingerprints. Nature, 387: 767 (1997). Secondary transfer -- from the point of primary transfer to a second object or person –“…in some cases, material from which DNA can be retrieved is transferred from object to hand.” Id.

13 Quantities of DNA Single nucleated cell—6 pg. (pico grams) Threshold of detection for ProfilerPlus— approximately 100 pg.

14 Taylor & Johnson Studies (1) A kisses B on cheek C touches B’s cheek with a glove DNA consistent with A and B found on glove

15 Taylor & Johnson Studies (2) A wipes his own face with a damp towel B wipes her face with same towel C touches B’s face with glove DNA consistent with A and B found on glove

16 People v. Robinson 7-11 Killers Leave Hat at Scene Ex-girlfriend implicates Robinson brothers Foreign DNA under victim’s fingernail does not match Robinson brothers DNA mixture on hat band “includes” Cory Robinson’s profile –But was the test interpreted correctly???

17 People v. Robinson (7-11 Killers Leave Hat at Scene)

18 People v. Robinson (how many contributors to the DNA on the hat?) If 2, Cory Robinson is excluded If 3 or more, Cory Robinson is included

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22 Strong Evidence of Three Contributors?

23 Unbalanced alleles of 2 contributors— NIST Mixture Study #2 Balanced alleles of same two contributors

24  Could two contributors have produced these results?  Two contributors produced these results

25 Jurors’ Reactions Battle of experts a draw Strong skepticism about defense challenge Unfortunate Interpretations –Fingernail DNA could be from hot dog –Low probability of evidence under prosecution theory “just an opinion” –Attack on statistics misconstrued at bogus attack on match –Peak height ratios indicate multiple contributors

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27 Strong Evidence of Three Contributors?

28 Stretching the Match Criterion: Profiler Plus Profiler Results D3S1358 vWA FGA Amel D8S1179 Glove 15,17(19) 15,17 20,22,24 XY 12,13,15 Defendant ,26 XY 13,15 Victim 15 15,17 20,22 X 12,13 D21S11 D18S51 D5S818 D13S317 D7S820 Glove 28,29,30,31 12,14,16,19 9,10,(11),13 8,12,14 7,9,11 Defendant 28,29 12,14 11,13 8,11 7,9 Victim 30,31 16,19 9,10 12,14 11

29 Stretching the Match Criterion: Profiler Plus Profiler Results D3S1358 vWA FGA Amel D8S1179 Glove 15,17(19) 15,17 20,22,24 XY 12,13,15 Defendant ,26 XY 13,15 Victim 15 15,17 20,22 X 12,13

30 Stretching the Match Criterion: Profiler Plus Profiler Results D3S1358 vWA FGA Amel D8S1179 Glove 15,17(19) 15,17 20,22,24 XY 12,13,15 Defendant ,26 XY 13,15 Victim 15 15,17 20,22 X 12,13 Extra Allele

31 Stretching the Match Criterion: Profiler Plus Profiler Results D3S1358 vWA FGA Amel D8S1179 Glove 15,17(19) 15,17 20,22,24 XY 12,13,15 Defendant ,26 XY 13,15 Victim 15 15,17 20,22 X 12,13 Missing Allele

32 Stretching the Match Criterion: Profiler Plus Profiler Results D21S11 D18S51 D5S818 D13S317 D7S820 Glove 28,29,30,31 12,14,16,19 9,10,(11),13 8,12,14 7,9,11 Defendant 28,29 12,14 11,13 8,11 7,9 Victim 30,31 16,19 9,10 12,14 11

33 Stretching the Match Criterion: Profiler Plus Profiler Results D21S11 D18S51 D5S818 D13S317 D7S820 Glove 28,29,30,31 12,14,16,19 9,10,(11),13 8,12,14 7,9,11 Defendant 28,29 12,14 11,13 8,11 7,9 Victim 30,31 16,19 9,10 12,14 11 Weak allele-- unbalanced loci

34 Stretching the Match Criterion: Profiler Plus Profiler Results D21S11 D18S51 D5S818 D13S317 D7S820 Glove 28,29,30,31 12,14,16,19 9,10,(11),13 8,12,14 7,9,11 Defendant 28,29 12,14 11,13 8,11 7,9 Victim 30,31 16,19 9,10 12,14 11 Missing allele-- unbalanced loci

35 Stretching the Match Criterion: Profiler Plus Cofiler Results D3S1358 D16S539 THO1 TPOX CSF1PO D7S820 Amel Glove 15,17 10,12 7,9,9.3 6, XY Defendant 17 10,11 7, ,12 7,9 XY Victim ,9.3 6, X

36 Stretching the Match Criterion: Profiler Plus Cofiler Results D3S1358 D16S539 THO1 TPOX CSF1PO D7S820 Amel Glove 15,17 10,12 7,9,9.3 6, XY Defendant 17 10,11 7, ,12 7,9 XY Victim ,9.3 6, X Missing allele

37 Stretching the Match Criterion: Profiler Plus Cofiler Results D3S1358 D16S539 THO1 TPOX CSF1PO D7S820 Amel Glove 15,17 10,12 7,9,9.3 6, XY Defendant 17 10,11 7, ,12 7,9 XY Victim ,9.3 6, X Missing alleles

38 Stretching the Match Criterion: Profiler Plus Cofiler Results D3S1358 D16S539 THO1 TPOX CSF1PO D7S820 Amel Glove 15,17 10,12 7,9,9.3 6, XY Defendant 17 10,11 7, ,12 7,9 XY Victim ,9.3 6, X Missing alleles

39 Statistical Issues The Prosecutor’s Question: Q:How likely is it that defendant would share so many alleles with the mixed sample on the glove if he is not a contributor?

40 Statistical Issues The Prosecutor’s Question: Q:How likely is it that defendant would share so many alleles with the mixed sample on the glove if he is not a contributor? A:The frequency of the shared alleles in the general population is extremely low, therefore it is unlikely that anyone other than the defendant was the second contributor.

41 Statistical Issues The Defense Attorney’s Question: Q:How likely is it that there would be so many discrepancies between the defendant’s profile and the mixed sample on the glove if defendant is a contributor?

42 Statistical Issues The Defense Attorney’s Question: Q:How likely is it that there would be so many discrepancies between the defendant’s profile and the mixed sample on the glove if defendant is a contributor? A: It is extremely unlikely that this test would fail to detect so many of the defendant’s alleles, and would detect the extra 19 allele, if defendant is the second contributor. Therefore, defendant is unlikely to be the second contributor.

43 Statistical Issues Likelihood ratio: p(E/DC) DC--Defendant is a contributor p(E/DNC) DNC--Defendant not a contributor

44 Statistical Issues Likelihood ratio: p(E/DC) DC--Defendant is a contributor p(E/DNC) DNC--Defendant not a contributor In order to estimate p(E/DC) we need extensive data about the operating characteristics of this test under the conditions encountered in this case.

45 Statistical Issues Likelihood ratio: p(E/DC) DC--Defendant is a contributor p(E/DNC) DNC--Defendant not a contributor Should we present this likelihood ratio to the jury?

46 Statistics Presented to Jury Frequency of matching profile (sometimes called the “Random Match Probability”) Likelihood ratio – L= p(data/D is source)/p(data/random man is source) No statistics –Mere match or “consistent” profiles –Identification “to a scientific certainty” Error rates?

47 What can make the prosecution’s statistics misleading? Failure to consider relatives Failure to consider potential for error Fallacious characterizations Inappropriate treatment of mixed sample comparisons Failure to account for “fudge factors” and flexible match criteria –A Daubert issue? see Risinger et al, Cal.LR. Jan. 2002

48 Statistical Fallacies The “prosecutor’s fallacy” –Equates frequency of matching DNA profile with probability suspect is “not the source” or probability “someone else” is the source The “defense attorney’s fallacy” The “false positive fallacy” See Thompson, Taroi & Aitken, How the Probability of a False Positive Affects the Value of DNA Evidence, Journal of Forensic Sciences 48 (1), January 2003.

49 Trawling for Matches Confirmation Case – DNA test identifies individual who is already a suspect and therefore is quite likely, a priori, to match Trawl Case – DNA test identifies individual who appears unlikely, a priori, to match

50 USING DNA TO TRAWL FOR KILLERS ; MORE POLICE DEPARTMENTS ARE STARTING MURDER PROBES BY ROUNDING UP HUNDREDS OF GENETIC SAMPLES. CIVIL LIBERTARIANS DECRY THE PRACTICE, WHICH THEY SAY TREATS EVERYONE AS A SUSPECT. --LA Times Headline, March 10, 2001 Detectives suspected the killer knew his victim. Yet the genetic profile could still belong to one of hundreds of her associates and neighbors. Investigators had a problem: Which one should they test for a possible match? The solution: Test them all. Fanning out across the city, detectives armed with Q-Tips sought voluntary mouth swabs from any men [the victim] knew in an all-out effort to find a genetic match.

51 Trawling May Magnify the Effects of Error Let’s assume error causes a false positive in one comparison in What, then, is the probability a reported match is a true match, rather than a false positive? –In the confirmation case? –In the trawl case?

52 Prior OddsRandom Match Probability False Positive Probability Posterior Odds 2:14x Billion:1 2:14x ,000:1 2:14x :1 Effect of a Reported DNA “Match” on the Odds That an Individual Is the Source of the Matching Sample From Thompson, Taroni & Aiken (2003)

53 Prior OddsRandom Match Probability False Positive Probability Posterior Odds 2:14x Billion:1 2:14x ,000:1 2:14x :1 1:10004x Million:1 1:10004x :1 1:10004x :1 Effect of a Reported DNA “Match” on the Odds That an Individual Is the Source of the Matching Sample From Thompson, Taroni & Aiken (2003)

54 Due process problems for those falsely identified in a trawl Revealing the trawl may inform jury of otherwise inadmissible prior bad acts Not revealing trawl may cause jury to overestimate strength of case DNA match may taint other evidence DNA match may create confirmatory bias in investigation Extended statute of limitations


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