by Written on behalf of Wise Health Law July 19, 2019 4 min read


It is a common practice in all forms of litigation to have experts involved. Sometimes they are retained to provide advice and guidance in the background, but most often one of the parties will want the experts to testify and provide “opinions” to the court which support their position. If you are asked to perform that role, what do you need to know before you accept such a retainer or agree to testify? It is understood that you may not have an option if you are summoned to appear.

Witnesses: The General Rule

The general rule is that witnesses are to testify about what they have seen, heard or read. They are not to draw inferences from the facts they attest to, nor are they allowed to give their opinions based on those events. The trier of fact is there to do that, be it judge or jury.

Exception for Expert Opinions

There is an exception to the general rule where expert evidence is offered on matters requiring specialized knowledge or skill. This makes sense as there will be issues before the court where the trier of fact is not equipped to draw true or accurate inferences and conclusions from the bare facts. This would be true in most health law matters.


An ongoing concern is the independence and impartiality of the expert. Are they providing to the court an unbiased and independent assessment by the standards of their profession or about a party's health issues? The worry is that they will have a natural inclination to favour the position of those who retain them, especially where they hope to be hired again or are dependant on such work for their livelihood.

How Does the Law Deal with Such Issues?

The issues involved in allowing a potential expert to testify were set out by the Supreme Court of Canada (SCC) in White Burgess Langille Inman v. Abbott and Haliburton Co. as follows:

To decide how our law of evidence should best respond to these concerns, we must confront several questions: Should concerns about potentially biased expert opinion go to admissibility or only to weight?; If to admissibility, should these concerns be addressed by a threshold requirement for admissibility, by a judicial discretion to exclude, or both?; At what point do these concerns justify exclusion of the evidence?; And finally, how is our response to these concerns integrated into the existing legal framework governing the admissibility of expert opinion evidence? To answer these questions, we must first consider the existing legal framework governing admissibility, identify the duties that an expert witness has to the court and then turn to how those duties are best reflected in that legal framework.

The First Step: The Threshold for Admissibility

The threshold for admissibility was established by the SCC in R. v. Mohan as follows:

Admission of expert evidence depends on the application of the following criteria:

  1. Relevance;
  2. A necessity in assisting the trier of fact;
  3. Absence of any exclusionary rule; and
  4. A properly qualified expert.

Further Discretion in the Court

Mohanalso underlined the vital role of trial judges in assessing whether otherwise admissible expert evidence should be excluded because its probative value was overborne by its prejudicial effect and provided a residual discretion to exclude evidence based on a cost-benefit analysis.

A further concern is to ensure that the dispute is resolved by the trier of fact and not the experts. The court strives to allow the trier of fact to make an informed judgment based on the evidence over a leap of faith in accepting at face value the opinions of the experts.

Finally, the court must carefully examine the admissibility of expert opinion when it is based on a novel or contested science by considering the reliability of that underlying science.

Expert evidence which does not meet the threshold tests should be excluded.

The Second Step: Seeking a Balance

Assuming the threshold tests are met, the adjudicator (usually a judge) must still balance the potential benefit of admitting the evidence against the risks of its prejudicial effect.

The Expert's Common Law Duty to the Court

There is a common law duty owed by all experts to the court. They must provide their evidence as an independent product of their skill uninfluenced by the exigencies of litigation. They must never assume the role of advocate. They must be seen to be unbiased, impartial and independent.

As was held in White Burgess, this common law duty is part of the admissibility analysis. The elements of this duty go to the admissibility of the evidence rather than merely to its weight. Secondly, there is a threshold admissibility requirement being independence and impartiality. The threshold for admissibility flows from this duty. The expert witnesses must assist the court, and this principle overrides their obligation to the party calling them. If an expert witness is unable or unwilling to fulfill that duty, they do not qualify to perform the role of an expert, and their evidence should be excluded.

Next week, we will continue the discussion and deal with the remaining issues.

At Wise Health Law, our lawyers rely on our significant experience before all levels of courts and tribunals to provide our clients with exceptional guidance and representation through the often-overwhelming litigation process. To find out more about how we can help, contact us online, or at 416-915-4234for a consultation.

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