Deference remains the due of the trial judge. The evidence was
The governing principles
 The principles governing the admissibility of expert opin-
ion evidence are well established and not in need of repetition.
Yet, some aspects of those principles merit brief mention in view
of the basis upon which the evidence was excluded and the errors
alleged to warrant reversal of that decision.
 To begin, the framework for the admissibility of expert
opinion evidence was established to guard against the dangers
posed to the litigation of disputes by the introduction of expert
evidence. This structure endeavours to ensure, as best it can, that
the criminal trial process does not degenerate into trial by expert,
and safeguards the ability of the trier of fact to critically assess
the evidence to determine whether the accused’s guilt has been
proven beyond a reasonable doubt: R. v. Bingley,  1 S.C.R.
170,  S.C.J. No. 12, 2017 SCC 12, at para. 13; White Burgess
Langille Inman v. Abbott and Haliburton Co.,  2 S.C.R. 182,
 S.C.J. No. 23, 2015 SCC 23, at paras. 17-18; R. v. Sekhon,
 1 S.C.R. 272,  S.C.J. No. 15, 2014 SCC 15, at para. 46.
Trial by expert is neither an attractive nor a necessary prospect.
 In their essence, rules of admissibility are exclusionary.
They exclude evidence that is both relevant and material. As
a rule of admissibility, the opinion rule excludes relevant and
material evidence. It does so because we leave it to the trier of
fact, not witnesses, to form opinions, to draw inferences and to
reach conclusions. We exclude opinions, those ready-formed inferences, because they are unhelpful to the trier of fact and might
even be misleading: White Burgess, at para. 14; R. v. Graat,
 2 S.C.R. 819,  S.C.J. No. 102, at p. 836 S.C.R.
 But we do not exclude every opinion expressed by a witness. For example, we recognize that some issues require special
knowledge, skill or expertise. We also realize that triers of fact are
not necessarily equipped to draw reliable inferences from facts
stated by witnesses. And so we allow witnesses to tell triers of fact
their opinions about these issues, provided that the witnesses are
qualified to do so: White Burgess, at para. 15; R. v. Abbey, 
2 S.C.R. 24,  S.C.J. No. 59 (Abbey ’82), at p. 42 S.C.R.;
R. v. Vassel,  O.J. No. 4512, 2018 ONCA 721, at para. 86.
 The structure for the admissibility of expert opinion evidence has two components. The first — the threshold stage —
imposes four basic requirements the proposed evidence must satisfy to qualify for reception. The second — the gatekeeper stage
— requires the judge to balance the potential risks and benefits of