The modern Canadian law on the admissibility of expert evidence began with the judgment of Sopinka J. in R. v. Mohan,  2 S.C.R. 9. But in the last two decades since Mohan was decided the law on expert evidence has changed significantly.
The test in White Burgess is now the governing test for the admissibility of expert evidence. It adopts a two-stage approach: the first stage focuses on threshold requirements of admissibility; the second stage focuses on the trial judge’s discretionary gatekeeper role. Each stage has a specific set of criteria.
STAGE ONE: Threshold Requirements for Admissibility
Expert evidence is admissible when It meets the threshold requirements of admissibility, which are:
a. The evidence must be logically relevant;
b. The evidence must be necessary to assist the trier of fact;
c. The evidence must not be subject to any other exclusionary rule;
d. The expert must be properly qualified, which includes the requirement that the expert be willing and able to fulfil the expert’s duty to the court to provide evidence that is:
ii. Independent, and
iii. Unbiased. [FN1]
e. For opinions based on novel or contested science or science used for a novel purpose, the underlying science must be reliable for that purpose, and
STAGE TWO: Gatekeeper Function: Do the Benefits of Admitting the Evidence Outweigh the Costs?
Expert evidence should not be routinely admitted with only its weight to be determined by the trier of fact. The unmistakable overall trend of the jurisprudence has been to tighten the admissibility requirements and to enhance the judge’s gatekeeping role”.
White Burgess, at para. 20.
The trial judge, in a gatekeeper role, determines that the benefits of admitting the evidence outweigh its potential risks, considering such factors as:
a. Legal relevance,
c. Reliability. [FN2]
d. Absence of bias.
In short, if the proposed expert evidence does not meet the threshold requirements for admissibility it is excluded. If it does meet the threshold requirements, the trial judge then has a gatekeeper function. The trial judge must be satisfied that the benefits of admitting the evidence outweigh the costs of its admission. If the trial judge is so satisfied then the expert evidence may be admitted; if the trial judge is not so satisfied the evidence will be excluded even though it has met the threshold requirements.
R. v. Abbey, 2017 ONCA 640.
FN1: an expert’s lack of impartiality and independence and an expert’s bias go to the admissibility of the expert’s evidence as well as to its weight, if admitted. At the admissibility stage these qualities are relevant to the threshold requirement of a properly qualified expert, and they are again relevant at the gatekeeper stage.
FN2: at the gatekeeper stage of admissibility the reliability of the proposed expert evidence is central to its probative value and thus to the benefits of admitting it: R. v. Abbey, 2009 ONCA 624 (CanLII), 97 O.R. (3d) 330, at para. 87 (Abbey #1).