Holy grail of mixture analysis? or How to frame a suspect?
- A simple way to calculate all mixtures
Comment – The case in which I participated is used only as an example.
The talk isn’t about that case, not about what happened or whether the verdict was correct.
(I don’t know).
Prolific Florida DNA consultant Dr. Kevin McElfresh employs as LR a number which he
calculates in very simple way.
If also correct, the method would achieve the “simple
yet valid” holy grail of mixture analysis.
It’s obviously, and dangerously, wrong.
The 2009 NRC report praised the practice of forensic DNA, especially
in comparison with the other forensic science modalities.
Experts in fingerprint, hair, or even bite mark would testify that a similarity
constituted very strong, even certain, evidence of identity.
Eventually it became clear that in effect the expert culture was to
quantify strength of evidence intuitively based on personal
Accumulation of examples and studies showed that expert
intuition exaggerates, errs, and works by confirmation
DNA escaped the same bad reputation because it quantifies. DNA
measures strength-of-evidence with the likelihood ratio (LR), a number
with a real, appropriate, and scientific meaning.
Reporting that number distinguishes DNA evidence from malarkey.
- “Informative” loci
Mixture evidence from the case
McElfresh defines “informative” as “all alleles expected
per prosecution hypothesis (i.e. suspect and victim) are present”
Perhaps the thinking is that they’re informative of what
you think you already know. Confirmation bias.
“Informative” panels are analogous to witnesses favorable to the
prosecution; tinted panels to alibi witnesses.
I disagree with his testimony that the tinted panel evidence, if calculated,
would be further prosecution-favorable evidence.
If that were right, DNA could never exculpate anybody!
- Close-up — total dropout
- Another locus
- Evidence changes your opinion
- Fingernail DNA evidence
The method will find positive evidence – LR>1 – for virtually anyone.
The murder was 30 years ago. Where was I at the time?
- I don’t recall
Honest: It’s tempting, as an experienced DNA hand looking at mixture
data and comparing it with a suspect profile, to make the judgement
that the suspect contributed.
Stop there and you’re like the old-time
hair-matching etc. experts — honest but dangerously unaware of your
Not honest: Add a patina of “science” with a calculation that lacks
logical basis, thus disguising the subjectivity as objective.
Active market: Among 1000+ cases of DNA assessment by eyeball
guesswork adorned by a meaningless number, I expect multiple
convictions of factually innocent suspects.