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The Role of Expert Evidence

by Christopher Bruce

This article was originally published in the summer 1999 issue of the Expert Witness.

The readers of this newsletter are familiar with the use of expert testimony in the Canadian court system. Nevertheless, most of us would be hard pressed to provide a clear definition of the difference between experts and lay witnesses. On a day-to-day basis, the best many of us could do would be to paraphrase the old saw, "an expert is what an expert does." On occasion, however, it may behoove counsel to examine closely the witness being put forward by opposing counsel and ask "is that individual truly an expert?" In this article, I summarise some of the leading decisions concerning this question from both the Canadian and American courts.

The classic Canadian statement of the role of expert evidence is found in Kelliher (Village of) v. Smith, ([1931] S.C.R. 672), in which the Supreme Court of Canada, quoting from Bevan on Negligence, concluded that in order for testimony to be considered "expert"

[t]he subject matter of the inquiry must be such that ordinary people are unlikely to form a correct judgment about it, if unassisted by persons with special knowledge. (p. 684)

Recently, in R. v. Mohan, ([1994] 2 S.C.R. 9, at 23) the Supreme Court elaborated on this requirement. There, Sopinka JJ stated that expert evidence must be both necessary in assisting the trier of fact and relevant. (Emphasis added)

Under the heading of "necessity in assisting the trier of fact" the Court made it clear that expert evidence was not to be admitted if the subject of the testimony concerned an issue which was within the common knowledge of the trier of fact. In particular, Sopinka JJ quoted approvingly from R. v. Turner, ([1975] Q.B. 834, at 841) in which Lawton, LJ concluded

An expert’s opinion is admissible to furnish the court with scientific information which is likely to be outside the experience and knowledge of a judge or jury. If on the proven facts a judge or jury can form their own conclusions without help, then the opinion of an expert is unnecessary. (R. v. Mohan, at 24)


…the evidence must be necessary to enable the trier of fact to appreciate the matters in issue due to their technical nature. (p. 23)

The Court ruled that, prima facie, expert evidence was "relevant" if it was "…so related to a fact in issue that it tends to establish it." (p. 20) However, that was not to be the only criterion. In particular,

Evidence that is otherwise logically relevant may be excluded … if it involves an inordinate amount of time which is not commensurate with its value or if it is misleading in the sense that its effect on the trier of fact, particularly a jury, is out of proportion to its reliability. (p. 21)

Furthermore, relevance was also to include a test to determine whether the evidence was "reliable" and "essential."

[E]xpert evidence which advances a novel scientific theory or technique is subjected to special scrutiny to determine whether it meets a basic threshold of reliability and whether it is essential in the sense that the trier of fact will be unable to come to a satisfactory conclusion without the assistance of the expert. (p. 25)

In short, the hallmarks of expert evidence were (a) that it concern matters of such a technical nature that the judge or jury could not be expected to reach a "correct" conclusion without assistance; and (b) that it be able to withstand close scrutiny to determine whether it was "reliable." But those with some experience with litigation will recognise that this decision left many issues unresolved. Most importantly, a number of the terms that were crucial to the application of the Court’s decision were not defined. Without definitions of terms such as "special knowledge," "reliability," "novel scientific theory," and "technical matters," the lower courts were provided with little direction concerning the characteristics of "expert" testimony.

Some insight into the issues which can arise, and how the courts might resolve them, may be obtained by reviewing the interpretation which the courts in United States have given to Rule 702 of their Federal Rules of Evidence:

Rule 702. Testimony by Experts

If scientific, technical, or other specialized knowledge will assist the trier of fact to understand the evidence or to determine a fact in issue, a witness qualified as an expert by knowledge, skill, experience, training, or education, may testify thereto in the form of an opinion or otherwise.

As the wording of this Rule reflects the wording chosen by the Supreme Court of Canada in R. v. Mohan, the issues faced by the courts in both countries are similar.

Two recent decisions of the United States Supreme Court – Daubert v. Merrell Dow Pharmaceuticals, Inc. ([1992] 509 U.S. 579) and Kumho Tire Co. v. Carmichael, ([1999] 131 F.3d 1433) have ruled on the interpretation of the terms "scientific, technical, or other specialized knowledge" contained in Rule 702. In Daubert the court set out four criteria for determining whether expert testimony met the requirement that it constitute "scientific knowledge." These are:

  1. Whether the theory or technique "can be (and has been) tested."
  2. Whether the "theory or technique has been subjected to peer review and publication."
  3. In the case of a particular technique, what "the known or potential rate of error" is or has been.
  4. Whether the evidence has gained widespread acceptance within the scientific community.

All of these criteria appear valuable for determining the admissibility of "scientific" evidence, such as the testimony of medical researchers. (The issue in dispute in Daubert was whether the drug Bendectin, when taken by pregnant women, had caused birth defects.)

These tests can also be applied to economic and psychological testimony, with some modifications. For example, the test of an economist’s or psychologist’s prediction that a particular child will graduate from university is not the usual "scientific" test, of waiting to see whether child does, in fact, graduate. Rather, it is a reference to the numerous statistical studies which have shown that a child’s ultimate educational attainment is significantly influenced by traits inherited from his or her parents and by such socio-economic factors as the child’s sex and his/her parents’ income and religion.

The second and fourth criteria are also applicable to testimony that is based on the use of theoretical constructs. For example, the concept of "opportunity cost," which is the basis for one of the methods of valuing household services, has been developed by economists. Although it would be difficult to find direct empirical "tests" of this hypothesis, and its "potential rate of error" is not known; it has been "subjected to peer review and publication" and has "gained widespread acceptance within the scientific community."

Similarly, whereas there is, to my knowledge, no published theoretical support for the use of the cross dependency approach to valuing fatal accident claims, a number of refereed articles provide such support for the use of the sole dependency approach. Again, although no "scientific evidence" can be offered that the latter approach is superior to the former, those who employ the latter can point to evidence of "peer review and publication."

The Daubert criteria proved less applicable to issues involving "technical" knowledge, such as that often proffered by engineers, however. Accordingly, the United States Supreme Court agreed to hear Kumho Tire. In that case, a number of passengers in the plaintiff’s vehicle were injured when a tire blew out. An expert in tire failure analysis relied in part on his own (extensive) experience to conclude that the blow out was caused by a defect and not by misuse on the part of the plaintiff. As the expert’s testimony did not meet any of the criteria set out in Daubert, the issue in Kumho was whether "technical and other specialized knowledge," as defined in Rule 702, was to be subjected to the same criteria as was "scientific knowledge." The Court ruled that it was not. Testimony about a technical matter could be considered to be "expert" if it

…. focuses upon specialized observations, the specialized translations of those observations into theory, a specialized theory itself, or the application of such a theory in a particular case.

The function of Rule 702 was not to restrict expert testimony to a narrow set of "scientific" disciplines, but to

… make certain that an expert, whether basing testimony upon professional studies or personal experience, employs in the courtroom the same level of intellectual rigor that characterizes the practice of an expert in the relevant field.

This requirement, that "intellectual rigor" be applied, offers one of the most important "gatekeepers" when evaluating the testimony of those who have been put forward as expert witnesses. Too often "experts" offer no justification at all for their choice of a particular approach; or they offer little more justification than that it has "always been done that way" or that "a number" of courts have employed that approach. This is not evidence of intellectual rigor; nor does it meet any of the criteria for reliability or relevance set out by the Canadian and American Supreme Courts.


Christopher Bruce is the President of Economica and a Professor of Economics at the University of Calgary. He is also the author of Assessment of Personal Injury Damages (Butterworths, 2004).


In this article Christopher Bruce examines the difference between experts and lay witnesses. He summarizes some of the leading decisions concerning "The Role of Expert Evidence" from both the Canadian and American courts.

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