Testifying Part 4 – Opinions of Experts
Original article: January 17, 2017 | By Sweta Tejpal | Reviewed and updated October 7, 2026
An expert witness can help a judge or jury understand matters requiring specialized knowledge. Qualifications alone do not make an opinion admissible or correct. The judge must decide whether the proposed evidence meets the legal requirements and whether its benefits justify the risks.
What makes someone an expert?
Expertise may come from study, training or practical experience. It must relate to the particular question the witness proposes to answer; a professional title does not confer expertise in every area. For example, collision reconstruction, forensic pathology and digital-forensic analysis may require different specialists. Each opinion must stay within the field and scope permitted by the court.
The Mohan admissibility requirements
In R. v. Mohan, [1994] 2 S.C.R. 9, the Supreme Court identified four threshold requirements:
Relevance to an issue the court must decide.
Necessity in assisting the judge or jury, rather than merely offering helpful commentary.
No other exclusionary rule preventing admission.
A properly qualified expert for the proposed subject.
An expert generally assists with matters beyond ordinary knowledge and experience. Novel or contested science, or science proposed for a new purpose, also requires scrutiny of its reliability for that purpose. Expert evidence must not displace the judge or jury’s responsibility to decide the case.
Meeting the threshold is not the end of the inquiry
White Burgess Langille Inman v. Abbott and Haliburton Co., 2015 SCC 23 explains a two-stage approach. After the threshold requirements are met, the judge weighs the benefits of receiving the opinion against risks such as unfair prejudice, confusion, unreliability and disproportionate use of trial time. An impressive witness or technical presentation does not remove the need for that assessment.
An expert’s duty is to the court
White Burgess also confirms that an expert must be able and willing to give fair, objective and non-partisan assistance. The opinion should reflect independent judgment, not the result the retaining party wants. Payment by a party does not automatically disqualify a witness, but relationships, interests and indications of advocacy require careful consideration. Remaining concerns may affect gatekeeping and the weight given to the evidence.
Examine the basis and limits of the opinion
For a proposed opinion in your case, discuss these questions with counsel:
What precise issue needs expertise, and what qualifications match it?
What observations, records and assumptions support the conclusion?
Are the method’s limitations, uncertainty and possible alternative explanations identified?
Would changing a disputed assumption change the answer?
Is a defence expert needed, or can the issue be addressed through disclosure, an admissibility application or cross-examination?
Admission and weight are different. An admitted opinion is still open to challenge; the court is not obliged to accept it simply because an expert gave it. A report should not be treated as proof of every fact it assumes.
Expert opinions and ordinary observations
Not every opinion requires an expert. A non-expert may sometimes summarize everyday observations in opinion form, subject to the applicable rules. See Part 5 — Non-Expert Opinion Evidence. Decide what evidence is needed with counsel early enough to consider reports, disclosure requirements and any necessary court application.
Continue the Testifying series
Part 4 — Opinions of Experts (this article)
Related guidance
For advice about your circumstances, contact our office. This article provides general information, not legal advice.