The Basics
Historically, expert evidence was not admissible at all, due in large part to concerns regarding the risk of the expert usurping the role of the trier of fact, and related concerns regarding necessity, reliability, bias, confusion, delay and cost. As the judiciary came to the realization that the fact-finding process could not be accomplished without the admission of expert evidence, rules were set up to limit the scope of that evidence.
In R. v Morris, [1983] 2 S.C.R. 190 Lamer J. (as he then was) set out the basic common law rules regarding the admissibility of evidence at trial (p. 201):
Thayer’s statement of the law which is still the law in Canada, was as follows:
“(1) that nothing is to be received which is not logically probative of some matter requiring to be proved; and
(2) that everything which is thus probative should comein, unless a clear ground of policy or law excludes it.“
To this general statement should be added the discretionary power judges exercise to exclude logically relevant evidence
“… as being of too slight a significance, or as having too conjectural and remote a connection; others, as being dangerous, in their effect on the jury, and likely to be misused or overestimated by that body; others, as being impolitic, or unsafe on public grounds; others, on the bare ground of precedent. It is this sort of thing, as I said before, – the rejection on one or another practical ground, of what is really probative, – which is the characteristic thing in the law of evidence; stamping it as the child of the jury system.”
The Court went on to outline the 4 criteria for the admission of expert evidence:
(a) relevance;
(b) necessity in assisting the trier of fact;
(c) the absence of any exclusionary rule; and
(d) a properly qualified expert.
With regard to relevance, the court stated:
Relevance is a threshold requirement for the admission of expert evidence as with all other evidence. Relevance is a matter to be decided by a judge as question of law. Although prima facie admissible if so related to a fact in issue that it tends to establish it, that does not end the inquiry. This merely determines the logical relevance of the evidence. Other considerations enter into the decision as to admissibility. This further inquiry may be described as a cost benefit analysis, that is “whether its value is worth what it costs.” See McCormick on Evidence (3rd ed. 1984), at p. 544. Cost in this context is not used in its traditional economic sense but rather in terms of its impact on the trial process. Evidence that is otherwise logically relevant may be excluded on this basis, if its probative value is overborne by its prejudicial effect, 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.
As to necessity, the Court stated:
This pre-condition is often expressed in terms as to whether the evidence would be helpful to the trier of fact. The word “helpful” is not quite appropriate and sets too low a standard. However, I would not judge necessity by too strict a standard. What is required is that the opinion be necessary in the sense that it provide information “which is likely to be outside the experience and knowledge of a judge or jury…the evidence must be necessary to enable the trier of fact to appreciate the matters in issue due to their technical nature.
The other 2 criteria are self-explanatory.
Mohan also left the door open to novel scientific evidence but stated that such evidence must be subjected to “special scrutiny” in order to avoid a distortion of the fact finding process. Drawing on the Daubert line of cases in the US, the court stated that novel scientific evidence must be evaluated in light of the following criteria:
(1) whether the theory or technique can be and has been tested:
Scientific methodology today is based on generating hypotheses and testing them to see if they can be falsified; indeed, this methodology is what distinguishes science from other fields of human inquiry.
(2) whether the theory or technique has been subjected to peer review and publication:
[S]ubmission to the scrutiny of the scientific community is a component of “good science,” in part because it increases the likelihood that substantive flaws in methodology will be detected.
(3) the known or potential rate of error or the existence of standards; and,
(4) whether the theory or technique used has been generally accepted:
A “reliability assessment does not require, although it does permit, explicit identification of a relevant scientific community and an express determination of a particular degree of acceptance within that community.”
