by Deborah Ikede & Nicole Berezin
Expert evidence can be decisive in litigation, often shaping the court’s assessment of liability, causation, and damages. For that reason, expert reports based on an incomplete or unreliable factual record warrant careful scrutiny. Common issues include an expert’s failure to review material records, undue dependence on an incomplete evidentiary foundation, or complete reliance on the subjective account of an unreliable or noncredible plaintiff/witness.
This raises a recurring strategic question for litigators confronted with a materially flawed expert report proceeding to trial: should the expert’s evidence be preliminarily challenged for exclusion at the beginning of the trial through a voir dire motion, or should the expert be allowed to testify with arguments as to the weight to be afforded to the opinion to be addressed in closing arguments?
The jurisprudence generally favours the latter approach. Courts typically are reluctant to exclude imperfect expert evidence at the threshold, preferring to admit it and then address any deficiencies through the weight ultimately assigned to the expert opinion and findings. Although exclusion remains available, it is only in the narrower category of cases where the defect expressly contravenes the established legal principles of admissibility. The first task therefore is to determine whether the defect(s) triggers issues relevant to weight or admissibility.
The Framework: Two Stages of Admissibility
The admissibility of expert evidence is assessed in two stages. The first threshold stage enquiry, guided by the test established in R. v. Mohan,[1] involves assessing whether the evidence is: 1) relevant 2) necessary to assist the trier of fact, 3) not subject to any applicable exclusionary rule, and 4) tendered through a properly qualified expert.
At the second, gatekeeping stage, as articulated in White Burgess[2] the trial judge weighs the evidence’s probative value against its prejudicial effect and may exclude otherwise admissible evidence where its risks to a litigant’s rights greatly outweigh any benefits it may have in assisting the trier of fact. Both stages are informed by rules 4.1.01 and 53.03 of the Rules of Civil Procedure, which require experts to be fair, objective, and non-partisan, with that duty prevailing over any obligation to the party that retained them.
The General Rule: Frailties Go to Weight
Most common deficiencies, including opinions based on an incomplete evidentiary record, reliance on a party’s uncorroborated self-report, or untested assumptions, do not usually prevent an expert from testifying. The settled rule is that weaknesses in the factual foundation of an opinion generally go to weight, not admissibility.[3] In practical terms, assuming the expert is properly qualified, if the opinion is reasonably supported by the factual premise on which the expert relies, their report will usually be admitted. Any unfounded or flawed assumptions, opinions or findings are then challenged through a rigorous cross-examination and considered by the trier of fact when deciding how much weight to give the opinion.
In practice, courts will often admit the report and then account for its shortcomings when deciding how much weight it deserves. For most flawed expert reports, the critical work is therefore done through a well-crafted cross-examination as opposed to a threshold admissibility challenge.
The Exception: When a Deficiency Warrants Exclusion
Exclusion remains available, but only where the deficiencies are serious enough to affect admissibility rather than merely diminish weight. The clearest examples are where it is glaringly obvious that the expert lacks the necessary impartiality,[4] where the opinion completely lacks any evidentiary foundation, or where its prejudicial effect exceedingly outweighs its probative value. Further, as emphasized by the Court of Appeal in Bruff-Murphy v. Gunawardena,[5] the trial judge’s gatekeeping role continues after an expert has been qualified and may require the evidence to be excluded or limited if the expert’s testimony ceases to be fair, objective, and non-partisan.
The Available Tools: From Pleadings to Cross-Examination
There are several tools available to litigants to challenge a flawed expert report. Some of which include: 1) retaining a responding expert to address the flaws in the expert report; 2) pre-trial motions to strike;[6] 3) voir dire motions; 4) targeted cross-examination during trial; and 5) drawing on evidence obtained during trial in closing arguments to further elucidate the errors in the expert report.
Exclusion or Weight: Choosing the Right Course
Where the concern is that the expert relied on incomplete records, contested assumptions, uncorroborated self-reports, or a questionable methodology, the issue will usually go to weight rather than admissibility. In those circumstances, the more effective response is often not a motion to exclude, but a focused and tactical cross-examination, or responding expert evidence.
By contrast, where the concern is more fundamental such as a material lack of impartiality, an opinion without an evidentiary basis, advocacy presented as expert opinion, or serious prejudice that impairs the court’s ability to rely on the evidence, a true admissibility issue may arise. In those cases, the appropriate response may include a pre-trial motion, a voir dire, or a documented reminder of the court’s continuing gatekeeping function.
The choice of whether to pursue exclusion or limiting weight matters. Seeking exclusion where the real issue pertains to weight can add unnecessary expense and may weaken a counsel’s credibility. Conversely, relying only on cross-examination where the defect truly engages admissibility may leave a necessary remedy unused and could negatively impact the outcome of the potential case.
What This Means in Practice
A flawed opposing expert report is rarely decisive on its own. Canadian courts generally prefer to admit expert evidence and allow its weaknesses to be tested through the adversarial process. As a result, most deficiencies in an expert report will go towards the weight the court accords the opinion, rather than its admissibility.
Effective advocacy therefore begins with identifying the true nature of the problem. If the flaw undermines only the strength of the opinion, it is usually best addressed through cross-examination, contrary evidence, and closing submissions. If the flaw is fundamental and triggers any of the entrenched admissibility principles, it might behoove counsel to consider whether the appropriate procedural response is moving to exclude the expert. The distinction matters because it not only informs the potential remedies available, but it also informs the most persuasive way to challenge the report.
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[1] R. v. Mohan, [1994] 2 S.C.R. 9.
[2] White Burgess Langille Inman v. Abbott and Haliburton Co., 2015 SCC 23, [2015] 2 S.C.R. 182 [“White Burgess”], drawn from R. v. Abbey, 2009 ONCA 624.
[3] R. v. Lavallee, [1990] 1 S.C.R. 852 at para. 65 applied in Ontario in Marchand v. Public General Hospital Society of Chatham (2000), 51 O.R. (3d) 97 (C.A.)
[4] As White Burgess, supra note 2 confirms, exclusion on the basis of partiality will be exceptional, not routine. This will generally involve cases where the expert moves from opinion evidence into advocacy.
[5] Bruff-Murphy v. Gunawardena, 2017 ONCA 502.
[6] White Burgess, supra note 2.