Admissibility of Expert Evidence - Malcolm Simmons


Experts can and often do give evidence of fact as well as opinion evidence. An expert witness, like any non-expert witness, can give evidence of what he or she has observed if it is relevant to a fact in issue.

Unlike other witnesses, an expert witness may also give evidence based on his or her knowledge and experience of a subject matter, drawing on the work of others, such as the findings of published research or the pooled knowledge of a team of people with whom he or she works. Such evidence also gives rise to threshold questions of admissibility, and the special rules that govern the admissibility of expert opinion evidence also cover such expert evidence of fact. There are many examples of a expert witnesses giving evidence of fact of that nature. For example, when an engineer describes how a machine is configured and works or how a motorway is built, he is giving expert evidence of factual matters, in which he or she draws on knowledge that is not derived solely from personal observation or its equivalent.

It is common in English criminal trials for the misuse of drugs for the Crown to adduce the evidence of a policeman who has the experience and knowledge to describe the quantities of drugs that people tend to keep for personal use rather than for supply to others.

In Myers, Brangman and Cox v The Queen[1], an appeal from Bermuda, the Judicial Committee of the Privy Council approved of the use of police officers, who had special training and considerable experience of the practices of criminal gangs, to give evidence on the culture of gangs, their places of association and the signs that gang members used to associate themselves with particular gangs. In giving such factual evidence an expert witness can draw on the general body of knowledge and understanding in which he is expert, including the work and literature of others. The expert witness must set out his qualifications, by training and experience, to give expert evidence and also say from where he has obtained information, if it is not based on his own observations and experience.

There are also other roles for the expert in the court process beyond giving evidence. Parties often employ experts to give advice or to brief counsel on the preparation of a cross-examination without using them to give evidence. They can also assist the court in its preparation for a technical case.

It is the duty of the party that seeks to adduce the evidence of an expert to assess whether he or she has the necessary expertise and whether the [1]  [2015] UKPC 40; [2015] 3 WLR 1145evidence is admissible. The party instructing the expert must disclose to the expert all of the relevant factual material which was to contribute to the expert’s evidence, including material which did not support the client’s case. The expert should disclose in the written report the relevant factual evidence so provided.


The Australian case of R v Bonython[1] gave relevant guidance on admissibility of expert opinion evidence. In that case King CJ[2] stated:

“Before admitting the opinion of a witness into evidence as expert testimony, the judge must consider and decide two questions. The first is whether the subject matter of the opinion falls within the class of subjects upon which expert testimony is permissible. This first question may be divided into two parts: (a) whether the subject matter of the opinion is such that a person without instruction or experience in the area of knowledge or human experience would be able to form a sound judgment on the matter without the assistance of witnesses possessing special knowledge or experience in the area, and (b) whether the subject matter of the opinion forms part of a body of knowledge or experience which is sufficiently organized or recognized to be accepted as a reliable body of knowledge or experience, a special acquaintance with which by the witness would render his opinion of assistance to the court. The second question is whether the witness has acquired by study or experience sufficient knowledge of the subject to render his opinion of value in resolving the issues before the court.”

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