What is the general rule?
That opinion evidence cannot be given in court.
Witnesses must testify to fact within their knowledge, not venture their own opinion.
What can a witness do though?
They can give evidence of the “impression produced on his mind at the time by what he observed”
What is the exception to the general rule?
Expert evidence - experts may provide an opinion on their topic of speciality, if the trier of fact needs such specialist assistance.
Kennedy v Cordia 2016
Leading authority, sets the test on whether expert opinion will be admissible:
- Will it assist the court?
- Do they have the necessary knowledge and experience?
- Are they impartial?
- Is there a sufficient body of knowledge to back their opinion up?
Will it assist the court?
No matter their skill, they cannot pronounce directly on the issue before the court.
(Davies) (Galletly v Laird)
Necessary knowledge and experience
Must be appropriately qualified, mostly will be formally but doesn’t have to be – relevant experience and knowledge will suffice.
They must have this otherwise it is inadmissible.
Impartiality of experts
An expert witness should provide independent assistance to the court by way of objective unbiased opinion.
Must provide factual basis for their opinion, and give an entirety of the evidence by commenting on facts that point the other way.
A sufficient body of knowledge from an established and respected science
Where an expert witness is to give an opinion based on certain facts, those facts must be established in evidence from a reliable science. (Young v HM Advocate)
Davies v Magistrates 1953
An expert witness cannot “usurp the function of the court”.
They must furnish the court with the necessary scientific criteria for testing the accuracy of their conclusions.
They should not simply make unsupported assertions.
Galletly v Laird 1953
An expert witness could not be asked whether certain materials were “obscene”, because that was the question which the court had to determine.
Jones v HM Advocate 2016
Prior involvement in a police investigation did not preclude an officer from being impartial in giving evidence in the resulting court case
HM Advocate v Grimmond 2002
Confirmed that expert witnesses cannot give evidence on matters of ordinary human nature and behaviour.
Instead, the jury could competently decide based on their own knowledge.
