Qualifying the Expert Witness
For expert evidence to be admissible the expert must be properly qualified. The party who tenders the witness as an expert is required to demonstrate that the witness has acquired a special knowledge of a particular topic by formal study, practical experience, or both, which extends beyond that of the trier of fact. The competence of the witness to give expert evidence, or to be qualified as an expert, does not depend on how the skill was acquired, only that it has been gained.
Justice Watt, Manual of Criminal Evidence, 2013 at pg. 428.
Deficiencies in an expert’s opinion go to weight, not admissibility.
R. v. Marquard, 1993 CanLII 37 (SCC) at pg. 224.
Additionally, in order for the expert to be properly qualified, the expert must be willing and able to fulfil the expert’s duty to the court to provide evidence that is:
ii. Independent, and
` iii. Unbiased.
Nothing less than scrupulous vigilance from trial judges is required when dealing with the admissibility of expert opinion evidence.
R. v. Livingston, 2017 ONCJ 645 (CanLII), at para. 36; see also White Burgess Langille Inman v. Abbott and Haliburton Co., 2015 SCC 23 (CanLII),  2 S.C.R. 182, at para. 12: “We are now all too aware that an expert’s lack of independence and impartiality can result in egregious miscarriages of justice.”
The court is required to consider the particular circumstances of the proposed expert and the substance of his/her proposed evidence.
The test 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.
White Burgess, at para. 50.
Given this test, the mere fact that the witness is a police officer, without more, will not result in exclusion.
For instance, R. v. T.A., 2015 ONCJ 624 (CanLII), at para. 20.
However, the nature and level of the proposed expert’s participation in the police investigation are factors which may give rise to a realistic concern of bias.
See R. v. Livingston, 2017 ONCJ 645 (CanLII).
Bias in its Various Guises
In R. v. France,  (SCJ), at para. 17 Malloy J. refers to a 2009 article by Professor David Paciocco (now Justice Paciocco of the Court of Appeal for Ontario) that discusses how expert bias can come in many forms:
Professor Paciocco stresses the importance of the expert maintaining an “open mind to a broad range of possibilities” and notes that bias can often be unconscious. He refers to a number of forms of bias: lack of independence (because of a connection to the party calling the expert); “adversarial” or “selection” bias (where the witness has been selected to fit the needs of the litigant); “association bias” (the natural bias to do something serviceable for those who employ or remunerate you); professional credibility bias (where an expert has a professional interest in maintaining their own credibility after having taken a position); “noble cause distortion” (the belief that a particular outcome is the right one to achieve); and, a related form of bias, “confirmation bias” (the phenomenon that when a person is attracted to a particular outcome, there is a tendency to search for evidence that supports the desired conclusion or to interpret the evidence in a way that supports it). Confirmation bias was a particular problem identified in the Goudge Report as Dr. Smith and other pathologists and coroners at the time approached their investigations with a “think dirty” policy, an approach “inspired by the noble cause of redressing the long history of inaction in protecting abused children,” and designed to “help ferret it out and address it.” Unfortunately, as commented on by the Goudge Report and by Professor Paciocco, such an approach raises a serious risk of confirmation bias.
The proponent of the evidence has the burden of establishing its admissibility
A person who opposes the admission of the evidence on the basis of bias has the burden of establishing a “realistic concern” that the witness is unwilling or unable to comply with the duty and the proffering party must rebut this concern on a balance of probabilities to satisfy the test for admissibility.
White Burgess, at para. 48.