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DNA Mixture Interpretations and Statistics –
To Include or Exclude
Cybergenetics © 2003-2013Cybergenetics © 2003-2013
Prescription for Criminal Justice ForensicsABA Criminal Justice Section
New York, NYNew York, NYJune, 2013June, 2013
Mark W Perlin, PhD, MD, PhDMark W Perlin, PhD, MD, PhDCybergenetics, Pittsburgh, PACybergenetics, Pittsburgh, PA
CHILD&
CHILD
CHILD&
CHILDCHILD
VICTIM
Television
Cabinet
Table
Table
L-Shaped Couch
Bathroom
Bed
room
Lau
ndry
Bed
room
StairsBookcase
VICTIM
VICTIM
Storage
Door to OutsideCloset
underpants
pajama pants
DNA mixture dataQuantitative peak heights at a locus
peak size
peakheight
Peak thresholds
Threshold
Under threshold, don't use alleles
Allele Pair7, 77, 107, 127, 14
10, 100%10, 12
10, 1412, 1212, 1414, 14
Simplify data for manual review
Allele dropout
Threshold
Under threshold, use alleles anyway
Allele Pair7, 77, 107, 127, 14
10, 10> 0% 10, 12
10, 1412, 1212, 1414, 14
Conjure phantom peaks
Manual interpretation issues
Evidence• call good data inconclusive• peaks are too low for methods• too many contributors to handle• potential examination bias
Database• hit by association, not by match• comparison: make false hits• restrict upload: lose true hits
TrueAllele® genotype model
Evidence• preserve data information• use all peaks, high or low• any number of contributors• entirely objective, no bias
Database• hit based on LR match statistic• sensitive: find true hits• specific: only true hits
Mixture weightSeparate mixture data into its contributor components
25% 75%
Genotype inferenceThorough: consider every possible genotype solutionObjective: does not know the comparison genotype
Explain thepeak pattern
Betterexplanationhas ahigher likelihood
Victim's allele pair
Another person's Another person's allele pairallele pair
Match information
Probability(evidence match)
Probability(coincidental match)
How much more does the suspect match the evidencethan a random person?
30x
3%
98%
TrueAllele inclusionFinds the right answer
11
Thresholds lose informationUnderstate the probative force
2 6
ThresholdStochastic Threshold
TrueAllele exclusionDoes not find the wrong answer
– 20
TrueAllele usage
• child molester got: 20 quadrillion match, 22 years• validation study: 4 peer-reviewed scientific papers• admissibility: Pennsylvania, California, N. Ireland• precedent: Pennsylvania appellate ruling• approval: NYS Commission on Forensic Science• cases: over 100 reports in criminal cases• trials: over 15 (rape, murder, terror, weapons, ...)
TrueAllele deployment
Casework systemInterpretation services
DNA truth, criminal justiceThis truth is incontrovertible. Panic may resent it, ignorance may deride it, malice may distort it, but there it is. - Winston Churchill
Police effective databaseProsecutor convict the guiltyDefender exonerate innocentCourt the whole truthCitizen prevent crime
Genotype models are more accurate. Who cares?
More information
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