“Opinion evidence is presumptively inadmissible. The party seeking to introduce it bears the onus of establishing its admissibility on a balance of probabilities: R. v. Millard, 2018 ONSC 4410. The test for the admissibility of expert opinion evidence was enunciated by the Supreme Court of Canada in R. v. Mohan, 1994 CanLII 80 (SCC), [1994] 2 S.C.R. 9 and further refined in White Burgess Langille Inman v. Abott and Haliburton Co, 2015 SCC 23 (CanLII), [2015] 2 S.C.R. 182.
Justice Penny Jones urged in The Children’s Aid Society, Region of Halton v. J.B. and D.T., 2018 ONCJ 884, at para. 12:
Given the real risk that a miscarriage of justice will arise from the too ready admission of unreliable expert opinion evidence into the trial record for consideration by the trier of fact (examples which are documented in the Goudge and Motherisk reports), trial judges, as gatekeepers, have been directed to actively scrutinize the credentials of the experts and the nature of the opinion evidence sought to be introduced at the front end by way of a voir dire. The purpose of doing so is clear—it is to weed out unreliable opinion evidence that would offer little benefit to the trial process given its unreliability and might distort and prejudice the fact-finding process. See R. v. Abbey, 2017 ONCA 640 (CanLII) [Emphasis added]
Justice Kathleen Baker in Ogwadeni: deo Six Nations Child Welfare v. K.L.H., 2021 ONCJ 339 (CanLII) at paras 18 and 19, succinctly summarized the two-stage process for the admission of expert evidence as follows:
(18) Expert evidence is admissible when it meets the test set out in the following two stage process:
First, the evidence must meet the threshold requirements of admissibility. Accordingly:
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, and
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.
(19) The court must then proceed to the second gatekeeping stage and weigh the benefits of admitting the evidence against the potential risks considering such factors as:
a. Legal relevance
b. Necessity
c. Reliability
d. Absence of bias
Where the Court is considering evidence from a proposed expert witness whose expertise is in novel science, “special scrutiny” of that science is required. The Court must determine whether the proposed evidence meets a basic threshold of reliability to be sufficiently probative such that one can conclude that it is necessary. In so doing, the Court must consider:
(a) whether the theory or technique can be and has been tested;
(b) whether the theory or technique has been subjected to peer review and publication;
(c) the known or potential rate of error or the existence of standards; and
(d) whether the theory or technique used has been generally accepted.
The function of expert witnesses is to provide the trier of fact with expert opinion evidence that is fair, objective and non-partisan: Bruff-Murphy (Litigation guardian of) v. Gunawardena, 2017 ONCA 502. In order to exclude expert evidence due to alleged bias or partiality, the evidence must be so tainted as to be rendered it of minimal or no assistance: Carmen Alfano Family Trust v Piersanti, 2012 ONCA 297 at para 111. An expert witness that has adopted the role of advocate for a party is not impartial: Bruff-Murphy (Litigation guardian of) v. Gunawardena, 2017 ONCA 502.
The Court should clearly define the subject area of a witness’ expertise and vigorously confine the witness’s testimony to it.”: Brandifferri v Wawanesa Mutual Insurance Co., 2011 ONSC 3200.”