Expert Opinion - Criteria. R v Sekhon
In R v Sekhon (Ont CA, 2014) the Court of Appeal took the oppourtunity to set out the criteria for the admission of expert testimony:
Requirements for Expert Opinion Evidence
 As set out R. v. Mohan, 1994 CanLII 80 (SCC),  2 S.C.R. 9, at pp. 20-23, and affirmed in R. v. J.-L.J., 2000 SCC 51 (CanLII), 2000 SCC 51,  2 S.C.R. 600, and R. v. D.D., 2000 SCC 43 (CanLII), 2000 SCC 43,  2 S.C.R. 275, the admission of expert evidence depends on the following criteria: 1) relevance; 2) necessity in assisting the trier of fact; 3) the absence of any exclusionary rule; and 4) a properly qualified expert.
 With respect to the “relevance” criterion, Mohan states that the judge must conduct a cost-benefit analysis to determine “whether its value is worth what it costs” (p. 21, quoting McCormick on Evidence (3rd ed. 1984), at p. 544). The cost-benefit analysis requires the judge to balance the probative value of the evidence against its prejudicial effect (Mohan, at pp. 23-24).
 As for the “necessity” criterion, Mohan holds that “[i]f on the proven facts a judge or jury can form their own conclusions without help, then the opinion of [an] expert is unnecessary” (p. 23, quoting Lawton L.J. in R v. Turner,  1 Q.B. 834, at p. 841). The Court went on to note that the concern “inherent in the application of this criterion [is] that experts not be permitted to usurp the functions of the trier of fact” (p. 24).
 Given the concerns about the impact expert evidence can have on a trial — including the possibility that experts may usurp the role of the trier of fact — trial judges must be vigilant in monitoring and enforcing the proper scope of expert evidence. While these concerns are perhaps more pronounced in jury trials, all trial judges — including those in judge-alone trials — have an ongoing duty to ensure that expert evidence remains within its proper scope. It is not enough to simply consider the Mohan criteria at the outset of the expert’s testimony and make an initial ruling as to the admissibility of the evidence. The trial judge must do his or her best to ensure that, throughout the expert’s testimony, the testimony remains within the proper boundaries of expert evidence. As noted by Doherty J.A. in R. v. Abbey, 2009 ONCA 624 (CanLII), 2009 ONCA 624, 97 O.R. (3d) 330, at para. 62:
The admissibility inquiry is not conducted in a vacuum. Before deciding admissibility, a trial judge must determine the nature and scope of the proposed expert evidence. In doing so, the trial judge sets not only the boundaries of the proposed expert evidence but also, if necessary, the language in which the expert’s opinion may be proffered so as to minimize any potential harm to the trial process. A cautious delineation of the scope of the proposed expert evidence and strict adherence to those boundaries, if the evidence is admitted, are essential. The case law demonstrates that overreaching by expert witnesses is probably the most common fault leading to reversals on appeal. [Emphasis added; citations omitted.] The trial judge must both ensure that an expert stays within the proper bounds of his or her expertise and that the content of the evidence itself is properly the subject of expert evidence.
 It is foreseeable that mistakes will be made and that, as happened in the instant case, testimony that strays beyond the proper scope of the expert evidence will be given. It is also foreseeable that defence counsel may fail to object to the testimony at the time the problematic statements are made. In a jury trial, once the statements have been made, it may be somewhat more difficult to address the problem — but a remedial instruction advising the jury to disabuse their minds of the inadmissible evidence will generally suffice. Judges, on the other hand, are accustomed to disabusing their minds of inadmissible evidence. It goes without saying that where the expert evidence strays beyond its proper scope, it is imperative that the trial judge not assign any weight to the inadmissible parts.