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Using Experts to Defend Your Criminal Charge

In a criminal trial, the Crown prosecution and your lawyer may call witnesses to provide evidence. Most witnesses are ordinary individuals, but an “expert witness” is a someone with sharp knowledge on a particular topic who can assist the court in understanding a specific issue. An expert witness delivers a professional opinion, while all other witnesses are confined to relaying facts within their sphere of knowledge, observation and experience.

Determining Expert Witnesses

The trial judge will determine if an individual is qualified as an expert and, if so, what the nature and scope of the proposed evidence will be. The judge will not permit the expert to provide an opinion on common matters in which he or she has no special skills, knowledge or training. Further, the evidence is expected to be impartial; there must not be any bias or appearance of bias.

Proving Expert Evidence

In order to prove expert evidence, the law requires that the following issues be given consideration:

  • Selecting the precise question with respect to which the opinion is sought;
    Ensuring the person is qualified to answer that precise question;
  • Determining the assumptions necessary to arrive at an opinion and ensuring these assumptions can be proven; and
  • Ensuring the expert opinion meets the admissibility requirements.

Admissibility of Expert Evidence

The admissibility of expert evidence depends on four “rules-based” criteria, set out in the case of R. v. Mohan, 1994:

  • The evidence provided must be both logically and legally relevant;
  • The evidence must be necessary to assist in drawing the correct inference;
  • The person giving the evidence must have the required qualifications of an expert;
  • There must be the absence of any other exclusionary rule; and
  • The admissibility of expert evidence also depends on the court’s cost/benefit analysis in its role as gatekeeper.

Calling a Witness to a Criminal Trial

Expert witnesses and other witnesses can be called upon by subpoena to testify. They may be required to appear in court to testify or produce documents related to the proceeding that are in his or her possession.

The defence is not obliged to call upon a witness but in the event that they do, the prosecutor has the right to cross-examine him or her. Each witness must answer all questions under oath, unless the Judge declares otherwise.



Mr. Gregory has the legal experience to know which cases require the use of experts and which do not. He has extensive professional knowledge and is an excellent communicator and negotiator with the Crown, defending clients across Western Canada for over 30 years.

Rod Gregory

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