4. Expert Witnesses
| Author | David M. Paciocco - Lee Stuesser |
| Profession | Justice of the Ontario Court of Justice - Professor of Law, Bond University |
| Pages | 190-206 |
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Expert opinion evidence is presumptively inadmissible. It can be admitted only if the party calling it satisfies the following four preconditions to admissibility, on the balance of probabilities:
· the expert evidence must be "necessary" in the sense that the expert deals with a subject-matter that ordinary people are unlikely to form a correct judgment about without assistance;
· the expert evidence must be logically relevant to a material issue;
· the witness must be qualified to offer the opinion in the sense that the expert possesses special knowledge and experience going beyond that of the trier of fact in the matters testified to; and
· the proposed opinion must not run afoul of any exclusionary rule apart entirely from the expert opinion rule ("the absence of an exclusionary rule")
Even if these four preconditions are met, the trial judge, as the "gatekeeper," must decide whether the expert evidence is sufficiently beneficial to the trial process to warrant its admission despite the potential harm to the trial process that may flow from the admission of the expert evidence.
In R. v. Mohan the Supreme Court of Canada held that the "[a]dmission of expert evidence depends on the application of the four following criteria": (1) necessity in assisting the trier of fact, (2) relevance, (3) a properly qualified expert, and (4) the "absence of an exclusionary
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rule" that would be offended by the admission of the evidence. The burden is on the party calling the evidence to establish that each of these components is satisfied, on the balance of probabilities.41The precise meaning of those four components, which reflect the prior common law,42will be explained below in this chapter. What is crucial to appreciate, however, is that these components are not to be applied as rigid, technical rules but rather as context-specific criteria for admissibility.43
In applying them the trial judge has an important gate-keeping function to perform to ensure that expert evidence is admitted only where warranted. Specifically, the judge is to evaluate whether the "costs" or problems caused by admitting the expert evidence would outweigh the benefits in doing so.44If so, the expert evidence will be inadmissible, in whole or in part.
As will be explained, the "benefits" of expert testimony mean nothing more than the probative value of the evidence. As for the "costs of admission," in R. v. D.(D.) the Supreme Court of Canada commented that these relate to the "distracting and time-consuming thing expert testimony can become."45Concern was expressed about the difficulty courts can have in understanding and evaluating expert evidence and the time and resources it can require. The Court also noted that the status or credentials of the expert, or the scientific or technical content of expert testimony, can cause triers of fact simply to adopt the expert evidence without adequate scrutiny.46The Mohan standards apply in civil litigation,47administrative cases where rules of evidence are applied,48and in criminal cases. They also
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apply in both jury and judge alone trials.49It is possible for one rule to operate in such varied kinds of cases because the gate-keeping component of the Mohan test makes its application highly contextual. Different outcomes can occur depending on the nature and forum of the litigation. In Pro-Sys Consultants Ltd. v. Infineon Technologies AG, for example, the British Columbia Court of Appeal applied a less rigorous admissibility evaluation in a class action case because disclosure under the Rules of Civil Procedure had yet to take place.50A human rights tribunal admitted expert evidence about racial profiling noting that the same evidence may not make the grade in a criminal case, because of the subtle nature of discrimination and the special role performed by human rights tribunals.51Even within criminal cases the intensity of the Mohan test vacillates. In light of the reluctance of criminal courts to deprive accused persons of defence evidence52courts are inclined to apply the standards of admissibility for expert evidence in a more generous fashion than for Crown evidence.53It follows that although prior decisions are useful as illustrations, whether evidence will satisfy the Mohan test is not a matter of strict precedent.54A brand of expert evidence admissible in one case may not be admissible in another. Admissibility is to be determined on a case-by-case basis, because the needs of the case or even the quality of the evidence presented about the expertise will vary. Moreover, the state of learning can progress. Expert techniques or science, such as DNA evidence, can gain trust as learning improves, while "with further study or experience [theories] may prove less reliable than once believed."55In
R. v. Burns, for example, the Supreme Court of Canada had approved of the admission of expert psychological evidence diagnosing a complainant as having been sexually assaulted, although no concerted challenge had been mounted to demonstrate the shortcomings of the theory sup-
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porting that diagnosis.56As a result, in R. v. Olscamp, Justice Charron, then a trial judge, did not consider herself bound on the question of whether similar evidence tendered in that case was admissible.57She examined the state of learning critically, and found that child abuse accommodation syndrome could not reliably diagnose sexually abused children. This same example also shows how expert evidence may be admissible when presented for one purpose but not another. In R. v. K.(A.) child abuse accommodation syndrome evidence was subsequently held to be inadmissible for the purpose of proving that the child was assaulted, but aspects of it were admissible in order to educate the jury that the very factors that the defence were relying on to show the complainant not to be credible could be nothing more than the sequelae of sexual abuse.58The Mohan standards therefore invite a highly contextual approach to the admission of expert evidence.
In R. v. Abbey the Ontario Court of Appeal felt the need to retool the admissibility test that had been articulated in Mohan.59This is because the Mohan standards include preconditions to admissibility that are necessary conditions to admission (necessity, relevance, the absence of any other exclusionary rule, and a qualified expert), as well as a gate-keeping function that requires a contextual examination and balancing of costs and benefits. Analytically, it is therefore better to undertake the analysis in two stages - preconditions to admissibility first and balancing second. Yet Mohan failed to separate them when describing the test.
Mohan authority had also developed a redundant structure for performing the gate-keeping function. In Mohan itself the Court seemed to contemplate that this cost-benefit analysis would be part of the "relevance" inquiry. Yet in R. v. D.(D.) the Court suggested that even the necessity component of the test required an evaluation of relative costs and benefits.60This seemed to replough the same ground as the relevance inquiry. Indeed, in R. v. D.(D.) the court went further and identified a residual "probative value/prejudice" discretion to exclude evidence that meets the Mohan test, even though the Mohan test itself will have already engaged these considerations during the cost-benefit analysis.61
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Abbey therefore invites judges to examine the four preconditions to admissibility first, without engaging in a contextual evaluation, and only then to apply the gate-keeping function of balancing competing interests. This adds to the clarity of the evaluation and avoids redundancy.
Given the rules of stare decisis the Ontario Court of Appeal cannot, of course, change the Mohan test. Only the Supreme Court of Canada can. Yet the Abbey approach is sensible, and to the extent that it simply reorganizes the same legal standards used by the Supreme Court of Canada in Mohan it is adopted here. The italicized proposition of law at the opening of this section on "Expert Witnesses" therefore sets out the Mohan standards but uses the Abbey structure.
The admissibility of expert evidence is determined during a voir dire.62
There are no rigid rules about how detailed or formal the inquiry must be. What is required will vary with the nature of the evidence in the case.63 To enable the judge to decide properly whether the proposed expert evidence will be admissible, it is essential for the party proposing the expert to indicate with precision what the scope and nature of the expert testimony will be and what facts it is intended to prove.64If admissibility is contested, information relating to each of the Mohan preconditions should be presented, as well as all information required to enable the judge to perform the gate-keeping function.
"The same understanding of necessity applies in both...
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