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The expert witness in court

Dr Niamh Nic Daéid

Reader in Forensic Science,

Centre for Forensic Science,

University of Strathclyde

Some key questions

Expertise .. What is it and how is it defined in a legal context ?

How is expertise used in court ?

How are expert opinions formed/evaluated and what effects this ?

How are expert opinions best communicated ?

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“Expert evidence may take the form of opinion on facts established by the expert or others, admissible by reason of his/her extensive knowledge and/or experience.”

Or

“…the expertness of his/her evidence may be essentially factual derived from his/her technical knowledge and ability in identifying the subject matter or as a result of careful analysis and/or observation.”

An expert must help the court by giving objective, unbiased opinion of matters within their expertise.

Criminal Procedure Rules

Lord Justice Auld , 2005

This raises some fundamental

Issues for scientific experts

and for some evidence types.

It is particularly problematic for

evidence involving an expression of the

likelihood of an event or other statistical

interpretations which are common in science

Expertise

‘Stockwell’ revisited: the unhappy state of facial mapping

A.Campbell-Tiech and A.Byrnes, Archibold News, Issue 6, June 24, 2005

With thanks to Ian Evett

Evidence given IN COURT by

4 different experts in

facial comparison from

different backgrounds

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Superimposition is dangerous and misleading

Superimposition is a good technique

Morphological examination can only be

undertaken by an expert with medical training Morphological examination does not

require any particular medical knowledge

Morphological examination is the only reliable

technique Morphological examination is hopelessly subjective

It has no scientific basis

Image enhancement is

Misleading and dangerous Image enhancement is indispensable

How can this happen ?

Is this ‘junk science’ ? If it is, why is it allowed in court ?

If it isn’t, why are the opinions of the experts so at odds with each other ?

Who makes the choice about admissibility and expertise ?

Expertise … not straight forward

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Assessment of evidential value

Probabilistic

reasoning Statistics

Hypothesis testing

Mathematical

systems Databases

Assessment of evidential value

Probabilistic

reasoning Statistics

Hypothesis testing

Mathematical

systems Databases

Source Sub source

Activity

Offence

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Either: the defendant left the crime stain

Or: some unknown person left the crime stain

What is the probability that the crime profile would match the defendant

if he had left it?

What is the probability that the crime profile would match the defendant

if some unknown person had left it?

Statisticians agree

that this is correctly termed

a match probability

MATCH

P(E|Hp)

P(E|Hd)

Evaluation of scientific information

LR

Relevance of evidence

Frequency of evidence

in the population

Random presence of

evidence

Frequency of evidence given

other scenarios of activity

Transfer of evidence

onto receptor

Persistence of

evidence on receptor

Recovery of evidence

P(E|Hp)

P(E|Hd)

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Observations

Issues the Jury

must consider

BALANCE

LOGIC TRANSPARENCY

ROBUSTNESS

With thanks to Ian Evett

MATCH

observation

Crime scene stain

suspect

I have observed the blood staining on the suspects jacket. The evidence is consistent with the blood having come from the suspect

The evidence is entirely consistent with the suspects involvement with the assault

A man is assaulted in the car park of a pub

A suspect is observed at the scene

His jacket is bloodstained

Evidence of the suspects involvement in the assault is required

LOGIC BALANCE

With thanks to Ian Evett

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MATCH

observation

Crime scene stain

suspect

I have observed the blood staining on the suspects jacket. The evidence is consistent with the blood having come from the suspect

The evidence is entirely consistent with the suspects involvement with the assault

In evidence, the suspect says that he saw the fight and gave

first aid to the victim and this is why there was blood on his jacket.

Errrr…. It’s consistent with

that explanation as well !!!

LOGIC BALANCE

With thanks to Ian Evett

So it follows that there is a 1 in a billion

probability that the crime stain had

come from someone else...

Hence it is almost certain that the

crime stain came from the defendant

The probability of finding that profile

if the crime stain had come from

someone else is 1 in a billion

ROBUSTNESS

The second statement does not follow from the first

This mistake was made in the Deen,

Doheny and G. Adams trials

The prosecutor’ fallacy

With thanks to Ian Evett

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R v. Sally Clarke, Court of appeal , Oct 2000

ROBUSTNESS

“…the chance of 2 children dying naturally in

these circumstances is very long odds indeed:

1 in 73 million”

The chance of 2 children being murdered in

these circumstances is also very small… and not articulated in court

The statistic was shown to be unreliable in appeal

With thanks to Ian Evett

TRANSPARENCY

What does this mean ??!!??

Can you understand it ??!!!

What would this convey to a jury ???

…professor xxxxx accepted that he could not totally

exclude the possibility that the hemorrhages were

consistent with death by asphyxia

He stated that “they are consistent with that because

I believe they might be there anyway”

R v. Sally Clarke, Court of appeal , Oct 2000 With thanks to Ian Evett

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Why should the jury believe you ?

...because I have attempted to place before the court all of the data and literature that may

assist the jury.

I can show the court where my opinion has been tested under controlled conditions

TRANSPARENCY

...because of my vast

experience in this field

...because I have done a thousand

cases like this one

With thanks to Ian Evett

Observations

Issues the Jury

must consider

BALANCE

LOGIC TRANSPARENCY

ROBUSTNESS

The evolution of scientific

reasoning in forensic science.

Logical framework

Databases

Literature and pool of knowledge

Reasoned opinion given opposing propositions

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Expert opinion

Observations

Issues the Jury

must consider

Juror’s common

sense

Expert opinion Judicial guidance

Juror’s appreciation

of the expert opinion

Critical cases:

Davies v Edinburgh magistrates R v Adams R v Deen R v Doheny R v Reed and Reed R v Weller R v T Daubert Frye

Science v Experience

Science v Technology

Controlling the pace

Controlling bias

Communication

Disclosure

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Key issues

Do Legal practitioners need to know about science or technology ?

How do we deal with bias and the suggestion of bias ?

How do we understand the subtitles of communication of science in the courts

Data without rigorous underpinning of published scientific research .. Developing legal confusion on this point

Thank you !

Dr Niamh Nic Daéid PhD,BSc,BA,FFSSoc,FICI,FHEA,FRSS,MRSC

n.nicdaeid@strath.ac.uk