R. v. M.C., 2014 ONCA 611:
 The law of evidence distinguishes between fact and opinion. The distinction seems more and ultimately one of convenience rather than of objective reality. All sensory data is mediated by our powers of perception, assimilation and expression. But the distinction remains and is expressed in the rule that generally, but not universally, excludes evidence of opinion.
 Among the exceptions to the opinion rule are the opinions of experts. Duly qualified experts may testify about subjects that are within their field of expertise, logically relevant to a material issue in the proceedings, and outside the experience and knowledge of the trier of fact: Mohan, at p. 20.
 Experts combine information accumulated from their own work and experience, marry it with evidence adduced through exhibits, admissions and the testimony of other witnesses in the proceedings, and express an opinion about a factual inference that should be drawn from the accumulated materials: Abbey, at para. 71. These opinions are evaluated by the trier of fact, like any other evidence admitted in the proceedings, in determining whether the allegations contained in the indictment have been proven beyond a reasonable doubt.
 The factual premise or construct to which the expert applies his or her expertise must be established, however, by evidence that is otherwise properly admissible in the proceedings. Mere reliance by the expert on a state of facts as the basis for his or her opinion does not amount to evidence much less proof of those facts. For example, a psychiatrist proffers an opinion about criminal responsibility based on an account of events provided to him or her by an accused. The account relied upon must be established by other admissible evidence: R. v. Abbey,  2 S.C.R. 24, at p. 46 (Abbey '82); R. v. Boucher, 2005 SCC 72,  3 S.C.R. 499, at para. 31; and R. v. D. (D.), 2000 SCC 43,  2 S.C.R. 275, at para. 55. Where the factual premise of the expert's opinion is not established by otherwise admissible evidence, the opinion is entitled to less, in some cases, to no weight:Abbey '82, at p. 46; and R. v. Lavallee,  1 S.C.R. 852, at p. 893.
 Deciding the admissibility of expert opinion evidence involves two steps. The first requires a determination of whether the conditions precedent to admissibility have been met. The second, only undertaken on satisfaction of the first, consists of a cost-benefit analysis that asks whether the benefits of receiving the evidence to a correct decision exceed the costs to the litigation process of admitting the evidence:Abbey, at para. 76.
 The criteria applicable to the first step in the admissibility inquiry are:
iii. absence of an exclusionary rule; and
iv. a properly qualified expert.
Mohan, at p. 20; and Abbey, at paras. 75 and 80.
 Among the Mohan criteria, two are of particular importance in this case: the absence of an exclusionary rule, and a properly qualified expert.
 The absence of an exclusionary rule refers to an exclusionary rule other than the opinion rule itself: Abbey, at para. 80. For example, expert opinion evidence of an accused's disposition must not contravene the bad character rule.
 Whether a proposed witness is a properly qualified expert about subject-matter that is properly the subject of expert opinion evidence is determined on avoir dire after counsel proffering the evidence has defined the nature and scope of the proposed opinion: Abbey, at paras. 62-63; R. v. Marquard,  4 S.C.R. 223, at pp. 242-244; and R. v. McIntosh (1997), 35 O.R. (3d) 97 (C.A.), at p. 104. The trial judge's task extends beyond deciding whether the proposed expert is qualified to proffer an opinion. The judge should also determine the nature and scope of the expert evidence and ensure that the expert's reach does not exceed his or her grasp or extend beyond boundaries established by the trial judge: Abbey, at para. 62; R. v. Sekhon, 2014 SCC 15, at paras. 46-47; and Marquard, at pp. 242-244.
 An expert may refer to and be cross-examined upon authoritative works in the field. But without express adoption of the content by the expert, the opinions expressed in the works cannot become expert evidence for the trier of fact to consider in reaching their decision: Marquard, at p. 251. A review, even an extensive review by an expert of literature in a related field does not, without more, permit the expert to proffer an opinion on a subject outside the area of the expert's field of expertise: Mathisen, at para. 126.
 Evidence that meets the Mohan criteria will not necessarily be received and available for consideration by the trier of fact. At the second stage, the trial judge exercises a "gatekeeper" function. The judge decides whether the expert opinion evidence is sufficiently beneficial to the trial process to warrant its admission despite the potential harm to that same process that may ensue from the introduction of that very evidence: Abbey, at para. 76. It is only where the benefits gained by the introduction of the evidence predominate over the costs associated with its introduction that the evidence will be admitted.
 To determine the "benefits" associated with the introduction of expert opinion evidence, the judge considers the probative value of the evidence, the significance of and the issue to which the evidence is relevant. Potential probative value includes an assessment of the reliability of the evidence, not merely of its subject-matter, but also the expert's methodology, expertise and objectivity: Abbey, at para. 87.
 On the "cost" side, the trial judge considers the consumption of time, prejudice and confusion. The trier of fact may not be adequately equipped to effectively and critically assess the evidence. The underlying material may be complex, the witness' jargon impenetrable, and the evidence resistant to effective cross-examination: Abbey, at para. 90.