Admissibility of Expert Opinion Evidence


In R. v. Abbey, 2009 ONCA 624 (CanLII), Doherty J.A. charted a two-step process for determining the admissibility of expert opinion evidence.

Step One—the Mohan Criteria

The four criteria from R. v. Mohan 1994 CanLII 80 (SCC), [1994] 2 S.C.R. 9 were absorbed under the first step:

(i)        Relevance,

Relevance as a prerequisite to admission under this step, refers to logical relevance. Legal relevance is reserved for consideration under the second step, where the court performs its gatekeeper function: Abbey, at paras. 82-85.

(ii)       Necessity in assisting the trier of fact,

Each case operates on its own facts. The assessment of necessity operates within the context of the facts

In R. v. J.(J.L.), 2000 SCC 51 (CanLII), [2000] 2 S.C.R. 600, at para. 56, Binnie J. explained that the very purpose of expert evidence is to “assist the trier of fact by providing special knowledge that the ordinary person would not know” and not to “substitute the expert for the trier of fact”. In R. v. Sekhon, 2014 SCC 15 (CanLII), at para. 45, Moldaver J. reinforced the court’s comments in Mohan, at p. 23: “if on the proven facts a judge or jury can form their own conclusions without help, then the opinion of [an] expert is unnecessary”.

(iii)      Absence of an exclusionary rule (other than the opinion rule),



(iv)      A properly qualified expert.



The party proffering the opinion must demonstrate on a balance of probabilities that these preconditions to admissibility exist before proceeding to the second step.



Step Two—the Gatekeeper Component

Provided the applicant satisfies the court as to the existence of the four Mohan criteria, then the court will go on to consider whether the proposed opinion evidence is sufficiently beneficial to the trial process to warrant admission, despite the “potential harm to the trial process that may flow from the admission of the expert evidence”: Abbey, at para. 76.

Step two is often referred to as the “gatekeeper” component of the expert evidence admissibility inquiry. The proposed evidence must meet all of these preconditions before being admitted for consideration by the trier of fact.





                                    R v B.W.W, 2017 ONSC 985 (CanLII).

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