The rules for qualifying an expert witness are generally well establish. An issue that has caught the attention of the courts recently is the “role” of an expert witness.
In Reginav. Dupe, 2009 ONCJ 320, the Justice did not qualify a Crown Police Expert witness on the issue of drug trafficking. The particular concern is the officer’s understanding of the role of an expert witness, which is fundamentally different from the great majority of tasks commonly performed by a police officer. During the voir dire, the expert testified that her role was to provide an opinion for the Crown. The representative passages of her testimony were captured in the judgment:
She responded, “It was put forward to me by the Crown what they were expecting an expert opinion on”. Mr. Smart asked what her instructions from the Crown were, and “what her understanding was about her role in this case”. Constable Cain responded, “To offer an expert [opinion] in cocaine, method of use, quantities and pricing, drug indicia, packaging and currency”. She readily agreed with defence counsel that her role in this trial was to “provide an opinion for the Crown”. The Crown asked no questions on this issue.
The Supreme Court of Canada in White Burgess Langille Inman v. Abbott and Haliburton Co., 2015 SCC 23 (CanLII),  2 S.C.R. 182, at para. 45.
I have already described the duty owed by an expert witness to the court: the expert must be fair, objective and non-partisan. As I see it, the appropriate threshold for admissibility flows from this duty…
… While I would not go so far as to hold that the expert’s independence and impartiality should be presumed absent challenge, my view is that absence such challenge, the expert’s attestation or testimony recognizing and accepting the duty will generally be sufficient to establish that this threshold is met.
Once the expert attests or testified on oath to this effect, the burden is on the party opposing the admission of the evidence to show that there is a realistic concern that the expert’s evidence should not be received because the expert is unable and/or unwilling to comply with that duty… [emphasis added.]
In this case, the court held that,
 I understand the inquiry on this point to operate in two stages: first, there should be admissible evidence on the voir dire that the proposed witness recognizes that their primary duty is to the court, and an express statement by them that they are willing to accept this duty during their testimony. Absent any particular challenge to such a statement, this aspect of admissibility will then be satisfied. This is where the Crown’s present application runs aground. I have no admissible evidence – either direct or by inference – that Constable Cain understands this primary duty. In fairness to her, the Crown did not ask any questions in this area. When the topic was broached in cross-examination, she testified that her role was to “provide an opinion for the Crown”. While it very well may be that she does understand the proper role of an expert witness, in the present circumstances (a relatively new drug officer from the same investigative unit that arrested the defendant), I am not willing to speculate on what her particular training and understanding in this area is. I leave it to future cases to explore Constable Cain’s particular understanding and suitability as an expert witness in this particular area.
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