6. The Presentation and Evaluation of Expert Evidence
| Author | David M. Paciocco - Lee Stuesser |
| Profession | Justice of the Ontario Court of Justice - Professor of Law, Bond University |
| Pages | 209-214 |
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Depending on the circumstances, there are a variety of ways in which expert evidence will be presented.
At times, expert witnesses simply provide general, background information to the trier of fact that is useful in assessing evidence. This can
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be done without the expert commenting on the particular case. For example, an expert might simply advise the trier of fact of such general matters as the phenomenon that abused children may well continue to associate with their abusers.159This information enables the trier of fact to generate its own conclusions based on the facts it finds.
Most often experts offer their interpretation of a given set of facts. At times the expert may be a witness to the facts forming the foundation for their opinion, such as the burn expert in R. v. Marquard who examined the burn before diagnosing it.160These witnesses will testify as eyewitnesses to the underlying facts, and then offer an expert opinion as to what those facts signify. As explained earlier, the expert evidence rules apply solely to the opinions offered. They do not apply to the testimony about the facts the expert has personally observed unless the observations in question could not have been made without expertise.161
At other times, the expert witness will conduct an investigation prior to trial that includes interviewing witnesses or reading documents, and then offer an opinion based on the facts they have "found." In other words, these expert witnesses rely in whole or in part on "hearsay" to form their opinions. In R. v. Lavallee, for example, the psychiatrist diagnosed the accused as suffering from battered woman’s syndrome based on conversations with the accused, her mother and consultation of hospital admission records.162Since it is essential for the trier of fact to know the basis for an expert opinion so that the opinion can be evaluated, it is permissible for expert witnesses to relate any inadmissible information that they have relied on, including hearsay. The inadmissible information that the trier of fact learns about in this way is to be used solely to enable the trier of fact to evaluate the opinion, but not as proof of facts.163In Lavallee the testimony of the expert about what he
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had been told about the accused’s history of abuse could not be used as proof of the events she and her mother had described when interviewed by him. The expert’s description of those conversations came in solely so that the basis for his opinions could be understood.
Paradoxically the fact that an expert opinion based on hearsay is admissible does not always mean that the trier of fact is entitled to rely on that opinion. The Supreme Court of Canada stated in R. v. Abbey that although an opinion based on inadmissible information may be received, "[b]efore any weight can be given to [the] expert’s opinion, the facts upon which the opinion is based must be found to exist" on the basis of admissible evidence.164In other words, while the expert can testify based on hearsay information, before the rier of fact may rely on that opinion there must be admissible evidence that proves that the hearsay information the expert witness relied upon in making that opinion was true. In this way there will be an evidentiary foundation for accepting the expert’s opinion.
This "rule in Abbey" was interpreted by some to mean that the entire foundation for an opinion had to be confirmed by independent evidence accepted by the trier of fact before that opinion could be used by the trier of fact. In R. v. Lavallee,165however, the rule in Abbey was explained to mean that although it is necessary for the facts supporting the opinion to be found to exist based on admissible...
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