The document discusses several key issues regarding the use of expert testimony in court: 1) What constitutes expertise and how is it defined in a legal context? Experts must provide objective, unbiased opinions within their expertise but cases show expert opinions can differ substantially. 2) How are expert opinions formed and evaluated, and what factors influence this? Expert opinions are not always robust and transparent, potentially misleading juries. 3) How can expert opinions best be communicated to juries to aid their understanding of complex scientific or technical issues? Effective communication is important as juries must consider expert testimony along with other evidence.

1. 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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2. Criminal Procedure Rules
Lord Justice Auld , 2005
“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
This raises some fundamental
Issues for scientific experts
“…the expertness of his/her evidence may be essentially
and for some evidence types.
factual derived from his/her technical knowledge and ability in
identifying the subject matter or as a result of careful analysis
and/or observation.” It is particularly problematic for
evidence involving an expression of the
An expert must help thelikelihoodby an event or other statistical
court of giving objective, unbiased
opinion of matters within their expertise.
interpretations which are common in science
Expertise
Evidence given IN COURT by
4 different experts in
facial comparison from
different backgrounds
‘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
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3. 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
Morphological examination is hopelessly subjective technique
It has no scientific basis
Image enhancement is
Image enhancement is indispensable Misleading and dangerous
Expertise … not straight forward
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 ?
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4. Assessment of evidential value
Hypothesis testing
Mathematical
Databases
systems
Probabilistic
Statistics
reasoning
Assessment of evidential value
Source Sub source
Activity
Offence
Hypothesis testing
Mathematical
Databases
systems
Probabilistic
Statistics
reasoning
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5. Evaluation of scientific information
Either: the defendant left the crime stain
MATCH
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
P(E|Hp) that this is correctly termed
P(E|Hd) a match probability
P(E|Hp)
P(E|Hd)
Relevance of evidence
Transfer of evidence
onto receptor
LR Frequency of evidence
Persistence of in the population
evidence on receptor
Random presence of
evidence
Recovery of evidence
Frequency of evidence given
other scenarios of activity
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6. Observations
BALANCE
LOGIC TRANSPARENCY
ROBUSTNESS Issues the Jury
must consider
With thanks to Ian Evett
Crime scene stain
BALANCE LOGIC
MATCH
A man is assaulted in the car park of a pub observation
A suspect is observed at the scene suspect
His jacket is bloodstained
Evidence of the suspects involvement in the assault is required
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
With thanks to Ian Evett
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7. Crime scene stain
BALANCE LOGIC
MATCH
observation
suspect
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.
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
Errrr…. It’s consistent with
that explanation as well !!!
With thanks to Ian Evett
ROBUSTNESS The prosecutor’ fallacy
The probability of finding that profile
if the crime stain had come from
someone else is 1 in a billion
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 second statement does not follow from the first
This mistake was made in the Deen,
Doheny and G. Adams trials
With thanks to Ian Evett
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8. 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 R v. Sally Clarke, Court of appeal , Oct 2000
TRANSPARENCY
…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”
What does this mean ??!!??
Can you understand it ??!!!
What would this convey to a jury ???
With thanks to Ian Evett R v. Sally Clarke, Court of appeal , Oct 2000
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9. TRANSPARENCY
Why should the jury believe you ?
...because of my vast
experience in this field
...because I have done a thousand
cases like this one
...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
With thanks to Ian Evett
The evolution of scientific
reasoning in forensic science.
Observations
BALANCE
LOGIC TRANSPARENCY
Logical framework ROBUSTNESS Issues the Jury
Databases
must consider
Literature and pool of knowledge
Reasoned opinion given opposing propositions
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10. Expert opinion
Observations
Juror’s common
sense
Expert opinion Judicial guidance
Juror’s appreciation
of the expert opinion Issues the Jury
must consider
Science v Experience
Critical cases:
Science v Technology
Davies v Edinburgh magistrates
R v Adams
R v Deen
R v Doheny
R v Reed and Reed
R v Weller Controlling the pace
RvT
Controlling bias
Daubert
Communication
Frye
Disclosure
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11. 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
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