Error in the likelihood ratio: false match probability

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Presentation transcript:

Error in the likelihood ratio: false match probability American Academy of Forensic Sciences February, 2017 New Orleans, LA Mark W Perlin, PhD, MD, PhD Cybergenetics, Pittsburgh, PA Cybergenetics © 2003-2017 Cybergenetics © 2017

Forensic evidence ? Suspects ? Rape kit

Likelihood ratio Pr probability E evidence H guilt ~H innocence X type variable x type instance p(x) prior probability q(x) posterior probability log logarithm

Forensic statistics log(LR) +6 Suspects -6 Rape kit

Presumption of innocence The principle that there is a presumption of innocence in favor of the accused is the undoubted law, axiomatic and elementary, and its enforcement lies at the foundation of the administration of our criminal law. Coffin v. United States (1895) Innocence hypothesis ~H follows prior probability p Match statistic distribution if presumed innocent, did not contribute to the evidence

Forensic statistic Likelihood ratio summarizes the evidence against a suspect summary LR = 3 log LR = ½ How much evidence is needed?

Beyond reasonable doubt False match probability (FMP) is the chance of error that an innocent person has at least this LR summary LR = 3 log LR = ½ error FMP = 1/100

Non-contributor distribution How is log(LR) distributed for innocent people?

Non-contributor #2

Non-contributor #3

Non-contributor #4 The log(LR) distribution for innocent people at one locus. At many loci?

Combine by convolution Joint LR is the product of independent locus values Distribution of a sum is the convolution of their distributions (for independent values) Total log(LR) is the sum of independent locus values

Non-contributor genotype

Stranger rape on New Year's Eve Southampton rapist Stranger rape on New Year's Eve

The DNA profile contained limited information. DNA rape kit evidence A trace amount of semen detected on vaginal swabs was submitted for DNA profiling The DNA profile contained limited information. A speculative search was done of the UK National DNA Database. The search found 13 profiles, 1 in the Hampshire area. Due to the low level and incomplete nature of the DNA profile, computer-based statistical evaluation was performed.

Database search

Evidence & error

TrueAllele YouTube channel More information http://www.cybgen.com/information • Courses • Newsletters • Newsroom • Presentations • Publications • Webinars http://www.youtube.com/user/TrueAllele TrueAllele YouTube channel perlin@cybgen.com