How Inclusion Interpretation of DNA Mixture Evidence Reduces Identification Information American Academy of Forensic Sciences February, 2013 Washington,

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Presentation transcript:

How Inclusion Interpretation of DNA Mixture Evidence Reduces Identification Information American Academy of Forensic Sciences February, 2013 Washington, DC Mark W Perlin, PhD, MD, PhD Kiersten Dormer, MS and Jennifer Hornyak, MS Cybergenetics, Pittsburgh, PA Thomas Meyers, MS and Walter Lorenz Allegheny County Medical Examiners Office, Pittsburgh, PA Cybergenetics ©

DNA Mixture Data Quantitative peak heights at a locus Allele value Allele quantity

Genotype Inference Computer-based probabilistic genotyping Explain the peak pattern Victim's allele pair Another person's allele pair Allele Pair 5%15, 15 90%15, 16 5%15, 17 15, 18 16, 16 16, 17 16, 18 17, 17 17, 18 18, 18 Thorough & objective

Identification Information Likelihood Ratio Explaining all the data under two competing hypotheses Matching genotype probability Evidence Coincidence 90% 9% LR = 90% 9% = 10 log(LR) = log(10) = 1 ban

Evidence Information Perlin MW, Belrose JL, Duceman BW. New York State TrueAllele ® Casework Validation Study. Journal of Forensic Sciences. 2013;58(6):in press. log(LR) Count Identification information (3.82)

Data Summary for CPI Over threshold, peaks are labeled as allele events All-or-none allele events Threshold Allele Pair 5%15, 15 9%15, 16 15%15, 17 9%15, 18 5%16, 16 15%16, 17 9%16, 18 13%17, 17 15%17, 18 5%18, 18

Information Loss Evidence Coincidence 9% LR = 9% = 1 log(LR) = log(1) = 0 ban Matching genotype probability Combined probability of inclusion CPI explains less of the data (no peak heights or model)

Evidence CPI Statistic Perlin MW, Belrose JL, Duceman BW. New York State TrueAllele ® Casework Validation Study. Journal of Forensic Sciences. 2013;58(6):in press. log(LR) Count Identification information CPI match statistic (3.82) 6.58 (0.80)

Mixture Information Study 16 cases 31 evidence items 41 genotype matches 2 & 3 person DNA mixtures Crime homicide (7) sexual assault (5) assault (2) death investigation (1) robbery (1) Item clothing (12) weapon (6) vehicle (5) skin swab (3) vaginal swab (3) fingernail (1) rectal swab (1)

Match Information vs. Statistic Computer-inferred genotype information (LR) Human review data summary statistic (CPI) 41 genotypes x 15 loci 615 locus experiments 517drop out or imbalance? human review? YESNO YES NO Computer-only results Computer & CPI

Information (per locus) log(LR) Count LR computer information (ban per locus) (0.590) N = 517

CPI Statistic (per locus) log(LR) Count LR computer information CPI human match statistic (0.615) (0.590) N = 517

Same Information Conservative Overstated Imaginative log(LR) information log(CPI) statistic Joint Statistic Distribution

rr2rr2 = = % 10.1% 72.3% log(LR) information log(CPI) statistic

Computer-only Results log(LR) Count YESNO Free of drop-out or imbalance? (0.664) N = 68 – (0.346) N = 29

Conclusions CPI does not correlate well with identification information CPI acts like a random positive number generator more loci give a higher statistic; not more information 28% of the time CPI overstates actual information lower error with the major of a two person mixture time to move on … probabilistic genotyping and LR CPI locus statistic relative to true information

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