Participant Expert vs Litigation Expert

The concept of evidence in everyday life is quite different from the special rules that govern evidence in a court of law.

We typically think of evidence in the simple terms of facts or information that support a belief or proposition. In the courts, however, there are rules around what can or cannot be introduced as evidence, how that evidence is presented and used, and the strength of some evidence over others.

One form of evidence that was recently clarified by the Ontario Court of Appeal has to do with evidence provided by participant experts versus litigation experts. This clarification has important consequences for determining what type of evidence is admissible at trial.

The Trial

Jesse Imeson was a former resident at Maryvale Adolescent and Family Services (Maryvale). Maryvale is a residential institution that provides both education and care to troubled youth.

Many years after his stay at Maryvale, Imeson was found guilty of three counts of second degree murder. Imeson served jailtime for those murders and, while serving his sentence, he alleged that he was sexually assaulted by a former employee of Maryvale.

Imeson took his claim to court where the trial judge ruled in his favor. However, the ruling was challenged in the Ontario Court of Appeal on the grounds that the evidence submitted by one of the key witnesses was not admissible.

The Issue: Participant Expert vs Litigation Expert

As the mental health clinician who treated Imeson, Dr. Kerry Smith was a key witness at the trial. He provided evidence as a participant expert.

Participant experts were once referred to as fact witnesses. In most cases, a participant expert can provide fact and limited opinion evidence.

Participant experts are different from litigation experts. Litigation experts are bound by Rule 53.03 of Ontario’s Rules of Civil Procedure. That rule outlines that any opinion evidence provided by a litigation expert must be based in an expert’s report. That report is to be provided to every party involved in the trial prior to its commencement and it must contain specific information outlined in Rule 53.03.

Dr. Smith was a participant expert and, as such, he was exempt from the same requirements that bind a litigation expert. His reports were allowed to be filed in their entirety, although they were based on notes taken during his sessions with Imeson and which were later destroyed. He was also permitted to provide opinion evidence regarding the sexual assaults, both in terms of whether or not they occurred and whether or not Imeson was injured by those assaults.

The Court ruled in favor of Imeson. A great deal of the evidence employed to come to this decision was the opinion evidence that came from Dr. Smith –  and herein lay the issue. At the Ontario Court of Appeal, the Court would weigh in on whether Dr. Smith’s opinions regarding the occurrence of the assaults and the subsequent harm went beyond his role as a participant expert because he was not a direct participant in those specific events.

The Appeal

On Appeal, the Court relied on the definition of participant expert determined in Westerhof v Gee Estate. It was in this case that the Ontario Court of Appealestablished the test relating to participant experts versus litigation experts:

[A] witness with special skill, knowledge, training, or experience who has not been engaged by or on behalf of a party to the litigation [who] may give opinion evidence for the truth of its contents without complying with rule 53.03 where:

  • the opinion to be given is based on the witness’s observation of or participation in the events at issue; and
  • the witness formed the opinion to be given as part of the ordinary exercise of his or her skill, knowledge, training and experience while observing or participating in such events.

Using this test as their basis, the Court of Appeal ruled that the evidence provided by Dr. Smith regarding whether or not the sexual assaults occurred and whether or not Imeson suffered harm should not have been permitted. The Court held that admitting Dr. Smith’s opinions and unredacted reports as evidence went beyond the limitations imposed on a participant expert as per the Westerhof test.


As a result of their findings, the Court ordered a new trial. The Court held that a new trial was necessary because a large percentage of the expert evidence provided at trial was inadmissible. More importantly, this clarification regarding the role of and limitations on a participant expert has a tremendous impact on admissible evidence in Ontario courts.

Specifically, this decision made it clear that opinion evidence provided by participant experts is only admissible if it’s based on their direct experience and participation. On the other hand, litigation experts may provide opinion evidence that goes beyond their experience and participation, as long as they prepare an expert’s report that complies with the guidelines in Rule 53.03.

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