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discretion to exclude evidence based on a cost-benefit analysis: p. 21. This is the
second component, which the subsequent jurisprudence has further emphasized:
Lederman, Bryant and Fuerst, at pp. 789-90; J.-L.J., at para. 28.
[20] Mohan and the jurisprudence since, however, have not explicitly addressed
how this “cost-benefit” component fits into the overall analysis. The reasons in
Mohan engaged in a cost-benefit analysis with respect to particular elements of
the four threshold requirements, but they also noted that the cost-benefit analysis
could be an aspect of exercising the overall discretion to exclude evidence whose
probative value does not justify its admission in light of its potentially prejudicial
effects: p. 21. The jurisprudence since Mohan has also focused on particular
aspects of expert opinion evidence, but again without always being explicit about
where additional concerns fit into the analysis. The unmistakable overall trend of
the jurisprudence, however, has been to tighten the admissibility requirements and
to enhance the judge’s gatekeeping role.
[21] So, for example, the necessity threshold criterion was emphasized in cases
such as D.D. The majority underlined that the necessity requirement exists “to
ensure that the dangers associated with expert evidence are not lightly tolerated”
and that “[m]ere relevance or ‘helpfulness’ is not enough”: para. 46. Other cases
have addressed the reliability of the science underlying an opinion and indeed
technical evidence in general: J.-L.J.; R. v. Trochym, 2007 SCC 6, [2007] 1
S.C.R. 239. The question remains, however, as to where the cost-benefit analysis
and concerns such as those about reliability fit into the overall analysis.
[22] Abbey (ONCA) introduced helpful analytical clarity by dividing the inquiry
into two steps. With minor adjustments, I would adopt that approach.
[23] At the first step, the proponent of the evidence must establish the threshold
requirements of admissibility. These are the four Mohan factors (relevance,
necessity, absence of an exclusionary rule and a properly qualified expert) and in
addition, in the case of an opinion based on novel or contested science or science
used for a novel purpose, the reliability of the underlying science for that purpose:
J.-L.J., at paras. 33, 35-36 and 47; Trochym, at para. 27; Lederman, Bryant and
Fuerst, at pp. 788-89 and 800-801. Relevance at this threshold stage refers to
logical relevance: Abbey (ONCA), at para. 82; J.-L.J., at para. 47. Evidence that
does not meet these threshold requirements should be excluded. Note that I would
retain necessity as a threshold requirement: D.D., at para. 57; see D. M. Paciocco
and L. Stuesser, The Law of Evidence (7th ed. 2015), at pp. 209-10; R. v. Boswell,
2011 ONCA 283, 85 C.R. (6th) 290, at para. 13; R. v. C. (M.), 2014 ONCA 611,
13 C.R. (7th) 396, at para. 72.
[24] At the second discretionary gatekeeping step, the judge balances the
potential risks and benefits of admitting the evidence in order to decide whether
the potential benefits justify the risks. The required balancing exercise has been
described in various ways. In Mohan, Sopinka J. spoke of the “reliability versus
effect factor” (p. 21), while in J.-L.J., Binnie J. spoke about “relevance, reliability
and necessity” being “measured against the counterweights of consumption of