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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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UCL. 14 Mar 05

“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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UCL. 14 Mar 05

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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UCL. 14 Mar 05

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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UCL. 14 Mar 05

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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UCL. 14 Mar 05

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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UCL. 14 Mar 05

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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UCL. 14 Mar 05

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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UCL. 14 Mar 05

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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UCL. 14 Mar 05

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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UCL. 14 Mar 05

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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UCL. 14 Mar 05

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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UCL. 14 Mar 05

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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UCL. 14 Mar 05

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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UCL. 14 Mar 05

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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UCL. 14 Mar 05

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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UCL. 14 Mar 05

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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UCL. 14 Mar 05

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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UCL. 14 Mar 05

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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UCL. 14 Mar 05

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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UCL. 14 Mar 05

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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UCL. 14 Mar 05

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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UCL. 14 Mar 05

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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UCL. 14 Mar 05

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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UCL. 14 Mar 05

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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UCL. 14 Mar 05

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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UCL. 14 Mar 05

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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UCL. 14 Mar 05

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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UCL. 14 Mar 05

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!