Expert evidence

Introduction to the use of expert evidence

General rule

At common law there is a general rule that witnesses must state facts not opinions. There are effectively two exceptions to this general rule: certain types of evidence given by non-expert witnesses and the evidence given by expert witnesses. 

Non-expert witnesses

A non-expert witness is allowed to express an opinion or impression where the facts perceived are too complicated or too evanescent in their nature to be recollected or separately and distinctly narrated. Some examples of matters on which a non-expert witness may state an opinion as a compendious way of stating facts are:

  • Estimations of speed and distance;
  • The identity of persons or articles;
  • The state of the weather;
  • The condition of articles;
  • The age of persons or articles¬†

Expert witnesses

An expert witness may rely on published and unpublished material in reaching conclusions, draw on his or her own experience and that of colleagues, and may refer to research papers, learned articles and letters during the course of giving testimony, such documents being themselves admitted in evidence and supporting any inferences which can fairly be drawn from them.

Admissibility of expert evidence

When will expert evidence be admissible?

The following conditions must be satisfied:

  • The matter must call for expertise; the inquiry has to be into a matter of art or science which is likely to be outside the experience and knowledge of the tribunal of fact. Expert help is therefore unnecessary on matters relating to normal human nature and behaviour.
  • The evidence must be helpful to the court in arriving at its conclusion;
  • There must be body of expertise in the area in question;
  • The particular witness must be suitably qualified as an expert in the particular field of knowledge; the question is whether the witness has sufficient skill and knowledge in relation to the field in question. There is no absolute requirement that the witness be professionally qualified, there being several areas where expertise is obtained through experience rather than study.
  • Permission to rely on the expert evidence must be obtained from the court.

Conflicts with expert evidence

Where there is a conflict between an expert and lay witnesses, generally the judge should refuse to accept the lay evidence in preference to uncontradicted expert evidence. However, the judge is not obliged to accept expert evidence if there are sufficient grounds for rejecting it, such as where it does not speak to a relevant issue, or where the judge does not believe the expert or is otherwise unconvinced by it.

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For more information on:

  • Obtaining facilities for inspection by experts
  • When will a stay of proceedings being ordered?
  • Legal professional privilege
  • Directions regarding expert evidence
  • Joint instruction of experts