R. v. McManus, 2017 ONCA 188:
(c) The Relevant Principles
 To be admitted into evidence, an expert's opinion must be (1) logically relevant to an issue in the proceedings; (2) necessary to assist the trier of fact; (3) not precluded by an exclusionary rule; and (4) tendered by a properly qualified expert:Mohan, at pp. 20-25. The trial judge also has residual discretion to exclude evidence that meets these criteria where the benefits of receiving the evidence are outweighed by the potential risks inherent in the introduction of the evidence: Mohan, at p. 21. This is the two-stage analytical structure for the admissibility of expert opinion evidence recognized by this court in R. v. Abbey, 2009 ONCA 624, 97 O.R. (3d) 330, at paras. 76-77, and by the Supreme Court in White Burgess Langille Inman v. Abbott and Haliburton Co., 2015 SCC 23,  2 S.C.R. 182, at paras. 22-24.
 In White Burgess, at para. 32, Cromwell J. described an expert's duty to provide opinion evidence that is fair, objective, and non-partisan:
Underlying the various formulations of the duty are three related concepts: impartiality, independence and absence of bias. 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.
 Cromwell J. held that an expert's lack of independence and impartiality must be considered at the "gatekeeper" stage, and "goes to the admissibility of the evidence in addition to being considered in relation to the weight to be given to the evidence if admitted": White Burgess, at para 45.
 An assessment of independence, impartiality, and bias is relevant to the fourth part of the Mohan test in determining whether the expert is properly qualified, and also factors into the balancing of benefit and risks of such evidence: White Burgess, at paras. 53-54; R. v. Shafia, 2016 ONCA 812, at para. 228. 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 Mohan test for admissibility: White Burgess, at para. 48.
 While an interest in or connection with the litigation does not automatically render the proposed expert evidence inadmissible, the court must consider whether the relationship or interest would result in the expert being unable or unwilling to carry out his or her primary duty to provide fair, non-partisan, and objective assistance: White Burgess, at paras. 49-50; R. v. Abbey, at para. 87. There is a heightened concern with police expert witnesses to ensure their ability to offer impartial expert evidence: see, for example R. v. Tesfai, 2015 ONSC 7792 (where measures taken to ensure the impartiality of the officer's opinion in a drug case were considered adequate) and R. v. T.A., 2015 ONCJ 624 (where the officer qualified to give expert opinion evidence was not involved in the investigation or the laying of charges and did not discuss his analysis with other officers involved in the project).
 Finally, the admissibility decision is entitled to deference on appellate review, unless there is an error in principle or the decision is unreasonable: Abbey, at para. 97; Shafia, at para. 234.
(d) The Principles Applied
 Regrettably, the trial judge did not properly consider the fourth Mohan criterion and did not fulfill his proper role as gatekeeper in respect of the proffered expert opinion. He did not give any real consideration to the question of D.C. Bullick's objectivity and ability to give independent and unbiased expert evidence and he did not consider the potential prejudice to the appellants if D.C. Bullick were allowed to testify in a dual capacity – both as an investigator as to several critical pieces of evidence forming part of the Crown's case and as an expert witness.
 D.C. Bullick's prior involvement with McManus and direct participation in the investigation and prosecution would immediately give rise to a concern about the officer's ability, despite his best intentions, to provide an impartial expert opinion. As explained in White Burgess, at para. 50, the question at the threshold stage 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." Further, "it is the nature and extent of the interest or connection with the litigation or a party thereto which matters": at para. 49. While D.C. Bullick may have testified that he was willing and able to provide an independent opinion, ultimately it was up to the trial judge to evaluate whether he could, in fact, do so.
 D.C. Bullick's position as a police officer did not disqualify him from giving expert evidence. However, the evidence elicited in the voir dire demonstrated that D.C. Bullick was neither independent nor impartial. D.C. Bullick testified that he had known McManus longer than four years, and that he believed McManus was a drug trafficker. He testified about being involved in investigations of McManus in the past that led to charges, and that he prepared his report in response to the preliminary inquiry judge's comment that the Crown's case was not strong. This testimony suggested that D.C. Bullick had a strong interest in seeing that McManus was convicted. His opinion could not help but be coloured by facts and information that did not form part of the trial record. Finally, his evidence was important and central to the Crown's case. In submissions on the voir dire, Crown counsel characterized it as filling in "an extremely important gap in the Crown's case", and later stated that the interpretation of the cellphone information was "very strong evidence and important evidence for the Crown."
 I am satisfied that there was a "realistic concern" that D.C. Bullick was unable to comply with the duty of an expert to provide independent, impartial and unbiased evidence, and that the Crown did not rebut this concern on a balance of probabilities, to satisfy the fourth Mohan criterion for admissibility: White Burgess, at paras. 48, 53.
 It is indeed surprising that the Crown sought to rely on the expert evidence of D.C. Bullick. The evidence that D.C. Bullick provided could easily have been offered by another witness who had no connection to the case. In fact, the Crown had already sought a report from an officer who had no involvement in the investigation who gave expert evidence at trial, without objection, on the interpretation of coded language in the documents alleged to be debt lists, all based on evidence she observed at trial.
 Instead of ruling the expert opinion evidence inadmissible, the trial judge left the issue of bias to be addressed in D.C. Bullick's cross-examination before the jury. In doing so, the trial judge failed to appreciate the practical impossibility that would present. To effectively explore the grounds of D.C. Bullick's bias and partiality, the defence would necessarily have elicited prejudicial bad character evidence about McManus before the jury.
Of the Law Societies of Upper Canada and Nunavut