Expert evidence is admissible when:
(1) It meets the threshold requirements of admissibility, which are:
a. The evidence must be logically relevant;
b. The evidence must be necessary to assist the trier of fact;
c. The evidence must not be subject to any other exclusionary rule;
d. The expert must be properly qualified, which includes the requirement that the expert be willing and able to fulfil the expert’s duty to the court to provide evidence that is:
ii. Independent, and
e. For opinions based on novel or contested science or science used for a novel purpose, the underlying science must be reliable for that purpose,
(2) The trial judge, in a gatekeeper role, determines that the benefits of admitting the evidence outweigh its potential risks, considering such factors as:
a. Legal relevance,
c. Reliability, and
d. Absence of bias.
Judges have an ongoing duty to ensure that expert evidence remains within its proper scope. It is not enough to simply consider the Mohan criteria at the outset of the expert’s testimony and make an initial ruling as to the admissibility of the evidence. The trial judge must do his or her best to ensure that, throughout the expert’s testimony, the expert stays within the proper bounds of his or her expertise and that the content of the evidence itself is properly the subject of expert evidence.: R v Sekhon, 2014 SCC 15 at para 46-47
An expert is entitled to give not only opinion evidence but also factual evidence – i.e., evidence based on first-hand observations: R v. Sheriffe, 2015 ONCA 880 at para 106
The "relevance" criterion
With respect to the “relevance” criterion, the judge must conduct a cost-benefit analysis to determine “whether its value is worth what it costs.” The cost-benefit analysis requires the judge to balance the probative value of the evidence against its prejudicial effect: R v Sekhon, 2014 SCC 15 at para 44
The "necessity" criterion
If on the proven facts a judge or jury can form their own conclusions without help, then the opinion of [an] expert is unnecessary. Inherent in the application of this criterion [is] that experts not be permitted to usurp the functions of the trier of fact. In other words, an expert cannot testify about matters going to the ultimate issue for the jury to decide: R v Sekhon, 2014 SCC 15 at paras 45-51
The "qualified expert" criterion
Expert witnesses have a duty to the court to give fair, objective and non-partisan opinion evidence. They must be aware of this duty and able and willing to carry it out. The expert’s opinion must be impartial in the sense that it reflects an objective assessment of the questions at hand. It must be independent in the sense that it is the product of the expert’s independent judgment, uninfluenced by who has retained him or her or the outcome of the litigation. It must be unbiased in the sense that it does not unfairly favour one party’s position over another. The acid test is whether the expert’s opinion would not change regardless of which party retained him or her. These concepts, of course, must be applied to the realities of adversary litigation.
Concerns related to the expert’s duty to the court and his or her willingness and capacity to comply with it are addressed initially in the “qualified expert” element of the Mohan framework. A proposed expert witness who is unable or unwilling to fulfill his or her duty to the court is not properly qualified to perform the role of an expert. If the expert witness does not meet this threshold admissibility requirement, his or her evidence should not be admitted. Once this threshold is met, however, remaining concerns about an expert witness’s compliance with his or her duty should be considered as part of the overall cost-benefit analysis which the judge conducts to carry out his or her gatekeeping role.
Absent challenge, the expert’s attestation or testimony recognizing and accepting the duty will generally be sufficient to establish that this threshold is met. However, if a party opposing admissibility shows that there is a realistic concern that the expert is unable and/or unwilling to comply with his or her duty, the proponent of the evidence has the burden of establishing its admissibility. Exclusion at the threshold stage of the analysis should occur only in very clear cases in which the proposed expert is unable or unwilling to provide the court with fair, objective and non-partisan evidence. Anything less than clear unwillingness or inability to do so should not lead to exclusion, but be taken into account in the overall weighing of costs and benefits of receiving the evidence.
The concept of apparent bias is not relevant to the question of whether or not an expert witness will be unable or unwilling to fulfill its primary duty to the court. When looking at an expert’s interest or relationship with a party, the question is not whether a reasonable observer would think that the expert is not independent. The question is whether the relationship or interest results in the expert being unable or unwilling to carry out his or her primary duty to the court to provide fair, non-partisan and objective assistance.
An expert’s prior connection with an investigation does not automatically disqualify that person from giving expert opinion evidence. This determination can only be made within the full context of the specific facts. Relevant factors will be the nature of the prior investigation, the role played by the individual expert in that investigation, and the nature of the proposed expert evidence. In most cases, suggestions that an expert witness lacks independence or impartiality will go to the weight of the expert’s evidence rather than its admissibility: R v Tang, 2015 ONCA 470 at para 6;R v McManus, 2017 ONCA 188
Expert witnesses are not barred from assisting the court with their special knowledge simply because they are not trained in the underlying science of the field. Such knowledge is required only where the science is novel. The purpose of the special rule for novel scientific evidence is to ensure that the reliability of the underlying technique or procedure used in forming the opinion has to be established by precedent, evidence, or statute: Bingley.
Weight of Expert Evidence
Where the expert’s opinions are based on statements made to the expert by others, those statements are not admissible for their truth. Where the factual premise of the expert’s opinion includes out-of-court statements made by others that are not established by otherwise admissible evidence, as for example by a listed or the principled exception to the hearsay rule, the opinion is entitled to less, and in some cases to no, weight: R v Sheriffe, 2015 ONCA 880 at paras 104-105
Where expert reports are based on conflicting hearsay statements by the accused (e.g., on an NCR hearing), the trial judge must attempt to reconcile them It is an error for the trial judge to smply assign such conflicting opinions no weight: R v Sualim, 2017 ONCA 178 at paras 26-38.
Reliance on Expert Evidence
A trial judge cannot abdicate to the expert witness his role in determining whether the accused was guilty of the offence. A trial judge's unquestioning acceptance of, and reliance on, an expert opinions bald opinion is a legal error warranting appellate intervention: R v Toole, 2017 ONCA 305 at para 11
Types of Experts
Drug Recognition Expert
The evidence of a Drug Recognition Expert is admissible without the need for a Mohan voir dire. The DRE’s expertise is not in the scientific foundation of the test but in the administration of the test itself: R v Bingley, 2017 SCC 12.
Where the probative value of an individual DRE’s evidence is diminished such that the benefits in admitting the evidence are outweighed by the potential harm to the trial process, a trial judge retains the discretion to exclude that evidence.
Furthermore, the determination of the DRE is not conclusive of the ultimate question of whether the accused was driving while impaired. It will always be for the trier of fact to determine what weight should be given to a DRE’s opinion, influenced by factors such as bias, failure to conduct the drug recognition evaluation in accordance with training, questionable inferences, bodily sample evidence, and evidence of bystanders or of other experts: Bingley at paras. 30-31.