Source: http://isthatlegal.ca/index.php?name=339-case-law
Timestamp: 2019-04-21 18:16:12+00:00

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In general terms, the admissibility of expert evidence is determined by the application of a two-step or two-stage process. The first step is concerned with the threshold requirements of admissibility. The second – the discretionary gatekeeping step – requires the judge to balance the potential risks and benefits of admitting the evidence: White Burgess Langille Inman v. Abbott and Haliburton Co., 2015 SCC 23 (CanLII),  2 S.C.R. 182, at paras. 19, 22-24; R. v. Sekhon, 2014 SCC 15 (CanLII),  1 S.C.R. 272, at paras. 43-44.
 Among the threshold requirements for the admissibility of expert opinion evidence is the absence of an exclusionary rule, other than the opinion rule itself. Usually, the exclusionary rule that intercedes is the character rule, which generally prohibits the Crown from introducing evidence of an accused’s bad character in proof of guilt: R. v. Mohan, 1994 CanLII 80 (SCC),  2 S.C.R. 9, at p. 25. But another exclusionary rule, such as the hearsay rule, could also intervene.
 A trial judge must take seriously the role of gatekeeper assigned by the authorities. And this is so at not only the second or gatekeeper stage, but also at the threshold stage and as the evidence is given: Sekhon, at paras. 46-47; R. v. J.(J.-L.), 2000 SCC 51 (CanLII),  2 S.C.R. 600, at para. 28.
 It is well established that expert opinion evidence may be founded, in whole or in part, on the basis of statements made to the expert by others. For example, a psychiatric opinion about criminal responsibility is frequently based, at least in part, on what an accused told the expert about relevant events. But in order for the out-of-court account to be admitted as evidence of the truth of what was said, that account must be established by admissible evidence: R. v. Abbey, 1982 CanLII 25 (SCC),  2 S.C.R. 24, at p. 46 (Abbey 1982). See also: R. c. Boucher, 2005 SCC 72 (CanLII),  3 S.C.R. 499, at para. 31; R. v. D.(D.), 2000 SCC 43 (CanLII),  2 S.C.R. 275, at para. 55.
 Where the factual premise of the expert’s opinion includes out-of-court statements made by others that are not established by otherwise admissible evidence, as for example by a listed or the principled exception to the hearsay rule, the opinion is entitled to less, and in some cases to no, weight: Abbey 1982, at p. 46; R. v. Lavallee, 1990 CanLII 95 (SCC),  1 S.C.R. 852, at p. 893.
 A final point has to do with the capacity of an expert to give evidence of firsthand observations that she or he makes that may be relevant to issues at trial. The opinion rule does not bar an expert from giving evidence of fact: Abbey 1982, at p. 42. Put another way, an expert is not confined by the opinion rule to expressing opinions only. The expert is entitled to give evidence of firsthand observations, including for example, those made during a psychiatric interview by a psychiatrist called to proffer an opinion on criminal responsibility.

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