Expert Opinion - Basics. R. v. Herlichka
In R. v. Herlichka (Ont CA, 2020) the Court of Appeal set out the basics of the admission of expert testimony:
 There is a two-stage framework for the admission of expert opinion evidence. At the threshold stage, four questions are asked: (i) relevance; (ii) necessity; (iii) the absence of an exclusionary rule; and (iv) the need for a properly qualified expert: White Burgess Langille Inman v. Abbott and Haliburton Co., 2015 SCC 23,  2 S.C.R. 182, at para. 19; R. v. Abbey, 2017 ONCA 640, 140 O.R. (3d) 40, at para. 48. . R. v. Biddersingh
 White Burgess, decided shortly after the ruling in this case, determined that at the first admissibility stage, and as part of determining whether the expert is properly qualified, the court must take into account the proposed expert’s ability to understand and to fulfill his or her duty to the court to provide impartial, independent and unbiased evidence: White Burgess, at para. 53; Abbey, at para. 48. The appellant emphasizes that expert evidence should only be excluded on that basis in “rare” and “very clear cases”: White Burgess, at para. 49; Mills, at para. 42.
 At the second admissibility stage – the gatekeeper stage – the trial judge exercises a residual discretion to exclude the evidence after having considered whether the benefits of admitting it outweigh its potential risks: Mills, at para. 44; Abbey, at para. 49. The trial judge must continue to take into account any concerns respecting the expert’s independence and impartiality at the gatekeeping stage: White Burgess, at para. 54; Mills, at para. 45. Accordingly, the expert’s impartiality, independence and willingness to provide an unbiased opinion is considered at both of the admissibility stages.
In R. v. Biddersingh (Ont CA, 2020) the Court of Appeal summarized the current state of the admission of expert evidence:
 In determining whether expert evidence is admissible, the trial judge must engage in the two-stage test adopted by the Supreme Court of Canada in White Burgess Langille Inman v. Abbott and Haliburton Co., 2015 SCC 23,  2 S.C.R. 182. At the first stage, the trial judge must determine whether the threshold requirements of admissibility are met: a) the evidence must be logically relevant; b) it must be necessary to assist the trier of fact; c) it must not be subject to any exclusionary rule; d) the expert must be properly qualified, including being willing and able to fulfil their duty to the court; and e) for any opinions based on novel science or science used for a novel purpose, the underlying science must be reliable: R. v. Abbey, 2017 ONCA 640, 140 O.R. (3d) 40, at paras. 47-48; White Burgess, at para. 23. If these requirements are met, the trial judge must advance to the second stage, in which they are required to fulfil a “gatekeeping role”: Abbey, at paras. 48, 53; White Burgess, at paras. 20, 24. As the gatekeeper, the trial judge must determine whether the benefits of the evidence outweigh its potential risks, considering such factors as legal relevance, necessity, reliability, and absence of bias: Abbey, at para. 48; R. v. J.-L.J., 2000 SCC 51,  2 S.C.R. 600, at para. 28.. Imeson v. Maryvale (Maryvale Adolescent and Family Services)
In Imeson v. Maryvale (Maryvale Adolescent and Family Services) (Ont CA, 2018) the Court of Appeal considers the use and admissibility of evidence from an expert under the combined Mohan/White Burgess doctrine:
 There are two steps in assessing the admissibility of expert evidence under the Mohan/White Burgess framework: White Burgess, at paras. 23-24.. R v M.C.
 The first step in determining admissibility is for the court to assess whether the proposed expert evidence meets the threshold requirements that the evidence is (i) logically relevant; (ii) necessary to assist the trier of fact; (iii) not subject to any other exclusionary rule; and (iv) proffered by a properly qualified expert who is willing and able to provide evidence that is impartial, independent, and unbiased: see White Burgess, at paras. 23, 53; Mohan, pp. 20-25.
 If the proponent of the evidence establishes the threshold for admissibility, the second discretionary gatekeeping step is for the trial judge to determine whether the potential benefits of admitting the evidence outweigh its potential risks to the trial process: White Burgess, at para. 24.
In this useful case of R v M.C. (Ont CA, 2014) the Court of Appeal took the oppourtunity to set out basic principles of the opinion exclusion rule:
The Opinion Rule. R v Vassel
 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, 1982 CanLII 25 (SCC),  2 S.C.R. 24, at p. 46 (Abbey ’82); R. v. Boucher, 2005 SCC 72 (CanLII), 2005 SCC 72,  3 S.C.R. 499, at para. 31; and R. v. D. (D.), 2000 SCC 43 (CanLII), 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, 1990 CanLII 95 (SCC),  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:
i. relevance;Mohan, at p. 20; and Abbey, at paras. 75 and 80.
iii. absence of an exclusionary rule; and
iv. a properly qualified expert.
 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 a voir dire after counsel proffering the evidence has defined the nature and scope of the proposed opinion: Abbey, at paras. 62-63; R. v. Marquard, 1993 CanLII 37 (SCC),  4 S.C.R. 223, at pp. 242-244; and R. v. McIntosh 1997 CanLII 3862 (ON CA), (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 (CanLII), 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.
In R v Vassel (Ont CA, 2018) the Court of Appeal considers the exclusionary 'opinion rule', of which expert evidence is an exception:
 The admissibility rule said to be applicable here is the opinion rule. Like its siblings, hearsay and character, the opinion rule is exclusionary by nature. It insists that witnesses give evidence of facts, not make statements of opinions or inferences drawn from those facts. It follows, at least as a general rule, that evidence of a witness’s opinion is not admissible: White Burgess Langille Inman v. Abbott and Haliburton Co., 2015 SCC 23 (CanLII),  2 S.C.R. 182, at para. 14. The most convincing rationale for the rule is that these ready-formed inferences offered by the witness are unhelpful to the trier of fact and might be misleading: R. v. Graat, 1982 CanLII 33 (SCC),  2 S.C.R. 819, at p. 836. See also White Burgess, at para. 14.
 As with other admissibility rules, the opinion rule is not unyielding. Not all opinion evidence is excluded. We recognize that, so far as matters that require special knowledge or skill are concerned, triers of fact, whether judges or jurors, are not necessarily equipped to draw true inferences from facts stated by witnesses. Assistance is necessary. And so it is that we permit witnesses to state their opinions about these subjects, provided the witnesses are shown to be experts in them: White Burgess, at para. 15; R. v. Abbey, 1982 CanLII 25 (SCC),  2 S.C.R. 24, at p. 42.
 Where the opinion rule is put forward as the ground upon which evidence proposed for admission should be excluded, the judge embarks upon a two-step inquiry into admissibility: White Burgess, at para. 22; see also, R. v. Abbey, 2009 ONCA 624 (CanLII), 97 O.R. (3d) 330, at para. 76, leave to appeal refused,  S.C.C.A. No. 125 (“Abbey ‘09”).
 At the first step of the admissibility inquiry, the proponent of the evidence must establish the threshold requirements of admissibility. These are the four Mohan factors:
iii. the absence of an exclusionary rule; and
iv. a properly qualified expert.
See White Burgess, at para. 23; R. v. Mohan, 1994 CanLII 80 (SCC),  2 S.C.R. 9, at p. 20; R. v. Bingley, 2017 SCC 12 (CanLII),  1 S.C.R. 170, at paras. 14-15.
 At this first step, the relevance factor refers to logical relevance: White Burgess, at para. 23; Abbey ‘09, at para. 84. The necessity requirement endeavours to ensure that the potential of expert opinion evidence to distort the fact-finding process is not lightly tolerated. White Burgess, at para. 21; R. v. D.D., 2000 SCC 43 (CanLII),  2 S.C.R. 275, at para. 46 and 57. As a result, the opinion evidence is unnecessary if on the facts established by other evidence the trier of fact can reach their own conclusion about the issue to which the proposed opinion evidence is directed without that opinion: Sekhon, at para. 45. What is required is that the opinion proposed for admission provide information that is likely to be outside the experience and knowledge of a jury: Mohan, at p. 23. Mere relevance or helpfulness is not enough: R. v. D.D., at para. 46.
 Under Mohan, a duly qualified expert is a witness who by study or experience has acquired special or peculiar knowledge of a subject about which a party proposes the witness will testify: Mohan, at p. 25. As we have seen, the extent of knowledge of the proposed expert must exceed the knowledge and experience of the trier of fact about the same subject: Bingley, at para. 22.
 It is the responsibility of the party who seeks to elicit expert opinion evidence from a proposed (or actual) witness to qualify the witness as an expert in the subject-matter about which the opinion is to be elicited: R. v. Marquard, 1993 CanLII 37 (SCC),  4 S.C.R. 223, at p. 243. It is the obligation of opposing counsel to object where appropriate and the task of the trial judge to ensure that the expert stays within the scope of his or her expertise: Marquard, at p. 244; Abbey ‘09, at para. 62; Sekhon, at paras. 46-47.
 At the second or gatekeeping step, the trial judge must balance the risks and benefits of admitting the evidence, thereby to determine whether the proposed evidence is sufficiently beneficial to the trial process to warrant its admission despite the potential harm to that same trial process that may flow from the admission of the expert evidence: White Burgess, at para. 24; Abbey ‘09, at para. 76. Relevance, necessity, and reliability, as well as the expert’s independence and impartiality, continue to play a role in weighing the overall competing considerations in admitting the evidence: White Burgess, at para. 54.
 Sometimes in giving evidence at trial, an expert may give opinions that extend beyond the subjects on which the expert has been qualified to give evidence. To the extent that an expert does so, the unqualified opinions are to be disregarded by the trier of fact. Where the trier of fact is a jury, the trial judge should instruct them accordingly: Marquard, at p. 244; Sekhon, at para. 48.
 A final point concerns anecdotal evidence, which sometimes enters the record of trial proceedings during the testimony of an expert. Anecdotal evidence is testimony that does not speak to the facts of the case but reasons from the witness’s prior experience to the probability of a particular result or occurrence in the case at hand. Anecdotal evidence is not legally relevant. Nor is it necessary. It lacks probative value. It is inherently prejudicial and tends to shift the onus of proof to an accused: Sekhon, at paras. 41, 49 and 50.