The Ontario Court of Appeal has recently provided direction on the ongoing role of trial judges with respect to the admissibility of expert evidence at trial. As noted by Justice Hourigan “The law regarding expert witnesses has evolved considerably over the last 20 years.…Today, expert witnesses are required to be independent, and their function is to provide the trier of fact with expert opinion evidence that is fair, objective and non-partisan.”
The plaintiff was injured in a motor vehicle accident. Liability was admitted. The jury trial dealt only with damages. At trial the plaintiff sought to exclude the evidence of Dr. Monte Bail, a psychiatrist retained by the defence. The plaintiff argued (1) Dr. Bail’s report was really just an attack on the plaintiff’s credibility, and (2) Dr. Bail was biased. The trial judge allowed the doctor was allowed to testify albeit with some restrictions on the scope of his testimony.
The Court of Appeal held a trial judge has consider two components when determining whether an expert’s evidence is admissible. The first component focuses on the Mohan requirements of (i) relevance, (ii) necessity, (iii) absence of an exclusionary rule, and (iv) a properly qualified expert. The second component is a “discretionary gatekeeping step” whereby a trial judge must balance probative value vs. prejudicial effect. As part of this second component a lack of independence or impartiality should be considered, not just with respect to the weight to be given to the expert’s evidence, but rather whether the evidence should even be admitted.
While Dr. Bail’s evidence may have met the threshold for admissibility, the Court of Appeal was of the view trial judge should have anticipated the possible issues with Dr. Bail’s impartiality simply by reviewing the medical legal report.
Further, it became apparent during his testimony that the doctor had assumed the role of an advocate for the defence. At that point the trial judge should have reassessed whether the doctor’s evidence could be relied on. In terms of the steps the trial judge should have taken the Court of Appeal stated,
 Where, as here, the expert’s eventual testimony removes any doubt about her independence, the trial judge must not act as if she were functus. The trial judge must continue to exercise her gatekeeper function. After all, the concerns about the impact of a non-independent expert witness on the jury have not been eliminated. To the contrary, they have come to fruition. At that stage, when the trial judge recognizes the acute risk to trial fairness, she must take action.
 Given this ongoing gatekeeper discretion, the question remains of what, as a practical matter, the trial judge could or should have done in this case. His first option would have been to advise counsel that he was going to give either a mid-trial or final instruction that Dr. Bail’s testimony would be excluded in whole or in part from the evidence. Had he taken that route, he would have received submissions from counsel in the absence of the jury and proceeded as he saw fit. Alternately, he could have asked for submissions from counsel on a mistrial, again in the absence of the jury, and ruled accordingly. In the event that he had to interrupt Dr. Bail’s testimony mid-trial, he would have had to consider carefully how best to minimize the potential prejudicial effect of the interruption from the respondent’s perspective.
 The point is that the trial judge was not powerless and should have taken action. The dangers of admitting expert evidence suggest a need for a trial judge to exercise prudence in excluding the testimony of an expert who lacks impartiality before those dangers manifest.
A new trial was ordered. The plaintiff was awarded costs of $22,000 for the appeal. The costs of the first trial were reserved to the new trial judge.