ucl monday 14 mar 2005
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UCL Monday 14 Mar 2005. Forensic inference – is the law a ass?. Ian Evett Forensic Science Service. The Forensic Science Service 2004. “If the law supposes that,” said Mr. Bumble... “the law is a ass – a idiot.” Charles Dickens: Oliver Twist ch 51. Match. - PowerPoint PPT PresentationTRANSCRIPT
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UCL. 14 Mar 05
UCLMonday 14 Mar 2005
Forensic inference – is the law a ass?
The Forensic Science Service 2004
Ian EvettForensic Science Service
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“If the law supposes that,” said Mr. Bumble... “the law is a ass – a idiot.”
Charles Dickens: Oliver Twist ch 51.
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Match
What is the probability that the crimeprofile would match the defendant if some unknown person had left it?
Either: the defendant left the crime stain
Or: some unknown person left the crime stain
What is the probability that the crimeprofile would match the defendant
if he had left it?
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What is the probability that the crimeprofile would match the defendant
if he had left it?
If the profile is not a mixture, thenthis probability is, effectively, one
It is important to bear in mind thatin more complex cases - such aspaternity and mixture cases - this
probability is substantially lessthan one and need careful consideration
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What is the probability that the crimeprofile would match the defendant if some unknown person had left it?
Statisticians agreethat this is correctly termed
a match probability
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DNA profiling is a model for the evolutionfor all scientific evidence
Ideally, we would like to see our opinionssupported by statistics in all cases.
However, it is clear that the courts arepoorly equipped to ensure that thestatistics are presented to the jury
in a method that best informstheir decision making.
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Match
DNA
The probability of finding that profile if the crime stain had come from
someone else is 1 in a billion
But the court is interested in theprobability that the crime stain
was left by the defendant
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That probability depends not just on the scientific evidence...
...but also on whatever otherevidence there is in the case
But the court is interested in theprobability that the crime stain
was left by the defendant
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There are various ways ofsetting the scientific evidence in
the context of a trial
The prosecutor’s fallacy
Doheny and Adams judgment
Probability of another person
Prior/posterior illustrations
The probability of finding that profile if the crime stain had come from
someone else is 1 in a billion
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Match
So it follows that there is a 1 in a billionprobability that the crime stain had
come from someone else...
The prosecutor’s fallacy
Hence it is almost certain that thecrime stain came from the defendant
This second statement doesnot follow from the first...
This mistake was made in the Deen,Doheny and G. Adams trials
The probability of finding that profile if the crime stain had come from
someone else is 1 in a billion
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Match
The likelihood ofit being anybody other than Alan
Doheny?
R v. Doheny
...I calculated the chance of finding allof those bands...to be about 1 in 40 million
Is about 1 in 40 million.
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Is it possible that the semen could have come from a different person from
the person who provided the blood sample?
I can estimate the chances of this semen having come from a man other than
the provider of the blood sample.
R v. G AdamsMatch
It is possible but it is so unlikely as toreally not be credible.
I can work out the chances as being less than 1 in 27 million
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MatchDoheny and Adams judgmentWhen the scientist gives
evidenceit is important that he should
notoverstep the line which
separateshis province from that of the
juryHe will properly explain to the Jury the nature of the match...
He will...give the Jury the random occurrence ratio – the frequency with which the matching DNA characteristics
are likely to be found in the population at large
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MatchDoheny and Adams judgment
Note that it is difficult to imagine a case in which it is reasonable to
consider every man, woman and child in the UK to be a possible perpretrator...
...nevertheless, let us see where this leads to
Provided he has the necessary data...it maybe appropriate for him then to say how many
people with matching characteristics...are likely to be found in the UK
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Match
How many other people in the UKare likely to have the same profile?
Approximately one twentieth of a person
It is difficult to believe that statements about fractions of people are helpful to the jury
Doheny and Adams judgment
The probability of finding that profile if the crime stain had come from
someone else is 1 in a billion
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Doheny and Adams judgment
Problems with the Doheny and Adams prescription
Conflicting evidence: eyewitness, alibi, geography, other scientific evidence
Concept of pool of equiprobable suspects
Fractional people
Relations of the defendant
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MatchDoheny and Adams judgmentWhen the scientist gives
evidenceit is important that he should
notoverstep the line which
separateshis province from that of the
juryThe scientist should not be asked his opinion on the
likelihood thatit was the Defendant who left
the crime stain,
nor when giving evidence should he use terminology which may lead the jury to
believe that he is expressing such an opinion
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Match
The evidence is a million times more probable if the first proposition is
true
Either: the defendant left the crime stain
Or: some unknown person left the crime stain
This is extremely strong support for the
first proposition
That statement is counter to the Doheny/Adams judgment.
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Either: the defendant wrote the questioned handwriting
Or: some unknown person wrote it.
In my opinion, there is extremely strong
support for the first proposition
Thank you, Dr Bloggs, that is very helpful.
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In my opinion, the crime mark and
control print were made by the same
person.
Thank you, Mr Holmes - that is amazingly helpful.
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R v. Dennis John Adams
Rape: Hemel Hempstead
DNA match between profile from vaginal swab and Mr Adams
Match probability1 in 200 million
Victim said that Mr Adamsdid not look like the rapist
Mr Adams had an alibi
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R v. Dennis John Adams
The central problem in this casewas that of combining statistically
based scientific evidence in supportof the prosecution...
...with non scientific evidence insupport of the defence
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There were two trials and twoappeals.
R v. Dennis John Adams
To cut a long story short...
...defence argued that the problem shouldbe approached by inviting the jury to quantify their assessment of the non-
scientific evidence
...then combining all of the various elementsby means of Bayes’ theorem
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R v. Dennis John Adams
Judgment:First appeal
It seems to us that the difficulties which arise in the present case stem
from the fact that, at trial, the defence were permitted to lead
before the jury evidence of the Bayes theorem.
But we have very grave doubt as to whether that evidence was properly admissible,
because it trespasses on an area peculiarly and exclusively within the province of the jury, namely
the way in which they evaluate the relationship between one piece of evidence and another.
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R v. Dennis John Adams
Judgment:First appeal
...the attempt to determine guilt or innocenceon the basis of a mathematical formula,
applied to each separate piece of evidence,is simply inappropriate to the jury’s task.
Jurors evaluate evidence... by the jointapplication of their individual commonsense and knowledge of the world...
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R. v. D.J. AdamsRetrial
Defence argued that the non-DNA evidenceshould be evaluated numerically.
All evidence should be combined using Bayes’ theorem
Questionnaire prepared andgiven to the jury
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R. v. D.J. AdamsRetrial
An example from thequestionnaire:
Bearing in mind that Ms Marley said that Mr Adams appeared to be in his early forties what is the probability
that she would say that her assailant was in his early twenties if Mr Adams were indeed the assailant?
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R. v. D.J. AdamsRetrial
Mr Adams was convicted.
Mr Adams appealed
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R v. Dennis John Adams
Judgment:Second appeal...the jury ...would have to ask themselves
whether they were satisfied that onlyX white European men in the UK wouldhave a DNA profile matching that of the
rapist who left the crime stain
It would be a matter for the jury, havingheard the evidence, to give a value of X.
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MatchThe probability of finding that profile
if the crime stain had come from someone else is 1 in 200 million
How many adultwhite males in
the UK would havethat profile?
Dennis Adams judgment
Approximately one tenth of a man
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R v. Dennis John Adams
X is approximately one tenth of a man
It would be a matter for the jury, havingheard the evidence, to give a value of X.
They would then have to ask themselveswhether they were satisfied that the
defendant was one of those men.
Well, that’s prettystraightforward, isn’t it?
Well, that’s prettystraightforward, isn’t it?
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R v. Dennis John Adams
...consideration of the case along the linesindicated would, in our judgment, reflecta normal course for a properly instructed
jury to adopt....
It is the sort of task which juries perform every day.
This appears to be a deliberateinstruction for juries to reason
irrationally
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The present position
If an expert is unable to carry out a statisticalassessment of the weight of the evidence*then courts allow him to express an opinion
of identity of source
* e.g. fingerprints, handwriting, toolmarks, footwear
If an expert is able to carry out a statisticalassessment of the weight of the evidence** then
courts will not allow him to express an opinion of identity of source - the statistic is put to the jury
** DNA profiling
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It is our hope, as forensic scientists,that expert opinions will increasingly
be based on databases and soundstatistical methods
The DNA experience suggests that ifthe weight of evidence can be measured
by a numerical statistic....
...then it is the statistic that should beput to the jury, rather than an expert
opinion of identity of source
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Most forensic scientists have no problem with this view...
Nevertheless, it would seem desirablethat we should explore with the judiciary
and the legal profession ways of presenting statistical evidence...
...then it is the statistic that should beput to the jury, rather than an expert
opinion of identity of source
...in a manner that is balanced, robust
and logical
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And present illustrations based on a range of priors agreed by the court
The most rational compromise would beto explain the meaning of the
likelihood ratio
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That’s allfolks!