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  • Zachary Mosher (UNB Law Student)

The Flexible and Principled Approach of the Gatekeeper Function: Judicial Discretion and Experts

Judicial discretion, also referred to as the “gatekeeper function”1, is a cost-benefit analysis a judge engages in to determine the probative/prejudicial effect of expert witness evidence.2 In R v. Mohan, the Supreme Court of Canada established four criteria for the admission of expert opinion evidence: relevance to the material issue, necessary to aid the fact-finder, it comes from a properly qualified expert, and its admission must not contradict any other exclusionary rule.3 R v. Abbey distinguished judicial discretion as a separate secondary step that safeguards against problematic evidence that passes the rule-based R v. Mohan criteria.4 Prior to R v. Abbey, the cost-benefit analysis was not as determinative because it was merely part of the relevance analysis. However, an important distinction Doherty J.A. makes in his reasoning for extracting the judicial discretion component from the original relevance analysis is that a cost-benefit analysis tests legal relevance, as opposed to the rule based relevance analysis which tests logical relevance.5 Now that judicial discretion has a prominent role in testing the admissibility of expert opinion evidence, the cost-benefit analysis sometimes dominates the answer to the question before the court. A good example comes from the New Brunswick case of R v. Edison, where defence counsel challenged a drug enforcement officer’s ability to be impartial, and whether the scope of subject matter he was to testify about was beyond the scope of his credentials.6 These types of questions are too complex for a rule based analysis to answer, whereas a discretionary test is more flexible and therefore more capable of considering the totality of circumstances surrounding an issue.

The implementation of the gatekeeper function in R v. Edison demonstrates how judicial discretion is able to test issues such as reliability that go beyond the assessment capable of a rule based approach. Furthermore, since the factors to consider in a cost-benefit analysis vary to adapt to the specific facts and circumstances of each case, the principle of stare decisis has far less weight attached to it. Overall, this case exemplifies how the separation of judicial discretion from the Mohan test represents a shift in evidence law from a rules based approach to a discretionary principled approach.

To determine whether S/Sgt Tomeo is a reliable witness, Ferguson J. extensively reviews S/Sgt Tomeo’s career and education as a drug enforcement officer, highlighting how a cost-benefit analysis is context specific.

The scope S/Sgt. Tomeo’s expertise, whether his testimony matches the same standards as in the professional field, the methodology he uses to craft his opinion, and his ability to be impartial, are all factors Ferguson J. must consider in determining reliability.7 These significant contextual factors place a heavy emphasis on reliability in the probative section of the cost-benefit analysis. The effect of introducing the reliability criteria into the second stage of the admissibility process is that Doherty’s J.A. separation of the probative/prejudicial analysis from the relevance analysis in the Mohan test also takes with it the reliability criteria. The importance of context shown in R v. Edison communicates part of Doherty’s J.A. critique of the Mohan test: that the issue of reliability requires a level of analysis that is too complex to be addressed in a rule based analysis. A reliability analysis fits better within the framework of judicial discretion because its emphasis on contextualizing the expert introduces various factors that are too exhaustive for a rule based analysis to comprehend. This shortcoming of the rule based approach demonstrates how the objective of the gatekeeper function to protect against unreliable expert witnesses also protects against mindless applications of the Mohan criteria.

In assessing the legal relevance of S/Sgt. Tomeo’s testimony on the subject matter of coded language, Ferguson J. departs from established precedent based on his cost-benefit analysis, establishing the testimony as legally relevant.

S/Sgt. Tomeo’s testimony on coded language raises the prejudicial issue of creating “the real potential to swallow the fact-finding function of the court”8. In addition, the cases of R v. Fougere 9 and Somerville v. R 10 were successfully appealed on basis of fact-finding interference. Yet, in view of the risk associated with admitting S/Sgt. Tomeo’s, Ferguson J. still chooses to admit S/Sgt. Tomeo’s testimony because he viewed S/Sgt. Tomeo’s opinion as essential to the trier of fact understanding the subject matter of the case. This determination establishes S/Sgt. Tomeo’s opinion as legally relevant. Doherty J.A. distinguishes logical and legal relevance in Abbey, explaining how logical relevance is a precondition to assessing legal relevance, which is why it occurs at the first level of the two step process. Legal relevance is whether a piece of evidence is “sufficiently probative” in spite of the prejudicial effect it may have.11 This weighing of the probative versus prejudicial effect is why the determination of legal relevance should be included in the second stage of the admissibility process. Based on this definition of legal relevance, Ferguson J places the probative value of necessity above the prejudicial effect of usurping the court’s fact-finding duty. Ferguson J. also recommends the trial judge follow examples given in the appellate decisions to help define the boundaries of the expert opinion.12 Judicial discretion may not eliminate the risk of expert testimony, but it does enable Ferguson J. to employ creative methods to mitigate the risk associated with it.

Ferguson’s J. choice to admit S/Sgt. Tomeo’s testimony focuses on the second step of determining admissibility of expert opinion evidence. The separation of the cost-benefit analysis from the Mohan criteria emphasizes the significance of reliability, impartiality, and the encroachment on the trier of fact, as factors to be considered in weighing the probative and prejudicial value of S/Sgt. Tomeo’s expert testimony. Shown through the framework of judicial discretion, this case demonstrates the benefit of a principled approach to address more subtle and complex issues such as reliability and legal relevance that a rule based approach is unable to answer.


1 R v Abbey, 2009 ONCA 78, 2009 CarswellOnt 5008 (WL Can).

2 Ibid at para 22.

3 R v Mohan, 1991 SCC 16, 1994 CarswellOnt 66 (WL Can).

4 R v Abbey, 2009 ONCA 78, 2009 CarswellOnt 5008 (WL Can). See also, R c J. (J.), 2000 SCC 61, 2000 CarswellQue 2310, where Binnie J. discusses how the probative/prejudice analysis helps ensure trial fairness.

5 Ibid at para 82.

6 R v Edison 2015 NBQB 4 and 9, 74 CanLII.

7 R v Abbey, 2009 ONCA 87, CarswellOnt 5008 (WL Can).

8 Ibid at para 76.

9 R v Fougere, [1988] NBJ No 17 (QL) (NBCA).

10 R v Somerville, 2012 NBCA, CarswellNB 83 (QL).

11 R v Abbey, 2009 ONCA 82, CarswellOnt 5008 (WL Can).

12 R v Edison 2015 NBQB 69, 74 CanLII.

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