Requirements of Expert Evidence

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General Principles

A trial judge must determine on a voir dire whether the individual is qualified as an expert, and if so, what the "nature and scope of the proposed expert evidence" will be.[1] The qualification process is one of delineating the boundaries of the evidence and the language used. [2]

The judge cannot permit the expert to give an opinion on common matters or matters that the expert has no special skills, knowledge, or training.

The qualification involves two phases. First, the adducing party "must establish the threshold requirements of admissibility" under the Mohan test.[3] Second, the judge must exercise discretion as a gatekeeper and balance the "potential risks and benefits of admitting the evidence".[4]

  1. R v Preeper and Doyle (1888), 15 SCR 401
  2. R v Abbey, 2009 ONCA 624 (CanLII)
    White Burgess Langille Inman v Abbott and Haliburton Co., 2015 SCC 23 (CanLII) at paras 22-24
    R v K. A., 1999 CanLII 3793 (ON C.A.)
  3. White, supra at para 23
  4. White, ibid. at para 24

Mohan Test

To qualify a person as an expert, the evidence must meet the Mohan requirements:[1]

  1. the opinion must be relevant;
  2. the opinion must be necessary to assist the trier-of-fact to draw the correct inference;
  3. the absence of any other exclusionary rule;
  4. the required qualifications of the proposed expert.

The Mohan test assumes that the area of science is not novel. Where the subject matter is novel, contested, or being used for a novel purpose, there is an additional requirement that the party adducing the evidence show that subject matter is reliable "for that purpose".[2]

White/Abbey Approach to Mohan
The preferred analytical approach to the Mohan test is to perform a two stage analysis.[3]

First Stage: Threshold Requirements
The first stage is to consider the "preconditions to admissibility".[4] The evidence must meet the threshold requirements of admissibility, which are:

  1. The evidence must be logically relevant;
  2. The evidence must be necessary to assist the trier of fact;
  3. The evidence must not be subject to any other exclusionary rule;
  4. 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:
    1. Impartial,
    2. Independent, and
    3. Unbiased.
  5. For opinions based on novel or contested science or science used for a novel purpose, the underlying science must be reliable for that purpose,

Should the evidence fail the requirements of the first stage, then the evidence is excluded.[5]

Second Stage: Gatekeeper
The second stage concerns a "gatekeeper inquiry" where the judge "must identify and weigh competing considerations to decide whether, on balance, admissibility of the evidence is favoured."[6] This second stage is effectively a cost-benefit analysis.[7] Factors to consider include:[8]

  1. Legal relevance,
  2. Necessity,
  3. Reliability, and
  4. Absence of bias.

Discretionary Exclusion of Qualified Expert Evidence
Qualified expert evidence can nonetheless be excluded if:[9]

  1. the evidence would tend to usurp the duty of the trier of fact
  2. the prejudicial effect outweighs the probative value
  3. the time required outweighs its probative value
  4. the cost required outweighs the probative value
  5. the influence of the evidence outweighs the evidence's reliability.

The proposed expert must have some indicia of scientific knowledge. It cannot be admitted if the witness is giving personal opinions based on their experience or knowledge from "some" literature and interviews.[10]

Expert evidence should be expected to be impartial to be admissible. There must not be any bias or appearance of bias.[11]

Weight Given to Expert Evidence
Where qualification is consented to by the other counsel, it does not permit the court to assign "weight beyond what the trial judge should otherwise attribute".[12] Weight is attributed by the court based on "reasons given for the expert opinion, the evidence bearing on the basis of the expert opinion and the extent of the expert’s expertise".[13]

Where the expert witness was not properly qualified and still gave evidence, their opinion may still be admissible absent objection from the opposing counsel.[14]

A failure for defence to make objections during trial are not likely obstacles to appeal since it is "foreseeable that defence counsel may fail to object to the testimony at the time the problematic statements are made".[15]

Expert Evidence Dangers
The courts "must be vigilant in monitoring and enforcing the proper scope of expert evidence" throughout the trial. Simply ruling on the Mohan criteria at the outset is not enough.[16]

Judge's must be cautious that the trial does not devolve into "trial by expert". The trier-of-fact must still be able to make "an effective and critical assessment of the evidence" that is based on "informed judgment" rather than blind faith in the expert's opinion.[17] To avoid this the courts as gatekeepers must watch out for:[18]

  • resistance to effective cross-examination by a non-expert counsel;
  • potential prejudice created by the expert's reliance on unproven material not subject to cross-examination;
  • the admission of "junk" science;
  • the risk of distraction from a "contest of experts";
  • an inordinate amount of time and money consumed by experts.
  1. R v Mohan, 1994 CanLII 80 (SCC), [1994] 2 SCR 9
    R v Sekhon, 2014 SCC 15 (CanLII) at para 43
  2. White Burgess Langille Inman v Abbott and Haliburton Co., 2015 SCC 23 (CanLII) at paras 23
    JLJ, supra at paras 33, 35 to 36, 47
    Trochym, supra at para 27
  3. R v Abbey, 2009 ONCA 624 (CanLII), 246 CCC (3d) 301, per Doherty J.A.
    R v Abbey, 2017 ONCA 640 (CanLII), at para 48
    R v Farnham, 2016 SKCA 111 (CanLII), at para 81
  4. Abbey (2009), supra at para 79
    Abbey (2017), supra at para 48
  5. Abbey (2017), supra at para 49 ("...if the proposed expert evidence does not meet the threshold requirements for admissibility it is excluded.")
  6. Farnham, supra at para 81
  7. Farnham, supra at para 81
  8. Abbey (2017), supra at para 48
  9. R v D.D., 2000 SCC 43 (CanLII), [2000] 2 SCR 275
  10. R v Bedford, 2000 CanLII 2487 (ON CA), (2000), 143 CCC 3d 311
  11. R v Docherty, 2010 ONSC 3628 (CanLII), [2010] OJ No. 2460 - father of defence counsel writes psych report, found invalid
  12. R v Strickland, 2013 NLCA 65 (CanLII)
  13. Strickland at para 19
  14. See Opinion Evidence Outside of Qualification below
  15. Sekhon, supra at para 48
  16. Sekhon, supra at para 46
  17. Abbott, supra at para 18
  18. Abbott, supra at para 18


Relevance require the judge to conduct a cost-benefit analysis to determine "whether its value is worth what is costs", which includes weighing the probative value against the prejudicial effect.[1]

The relevance requirement includes both logical relevance (the relationship between the evidence and the fact in issue it is being used to establish) and legal relevance (the probative value).[2]

Expert evidence can be relevant where it establishes motive to commit the offence.[3] It can also be relevant ot demonstrate animus towards the victim.[4]

It is not legally relevant for a qualified drug expert to say that they have never encountered a blind drug courier in their investigations.[5]

  1. R v Sekhon, 2014 SCC 15 (CanLII) at para 44
    R v Mohan, 1994 CanLII 80 (SCC), [1994] 2 SCR 9 at pp. 23-24
  2. R v K.(A.), 1999 CanLII 3793 (ON CA), (1999) 137 CCC 3d 225 (ONCA)
  3. R v Ma, [1978] O.J. No. 1425(*no CanLII links)
    R v Boucher, 2000 CanLII 6087 (QC CA)
    R v Wilson and Boswell, 2002 CanLII 49653 (ON SC), (2002), 166 CCC (3d) 294 (Ont. S.C.J.)
  4. R v McLeod, [1982] O.J. No. 59(*no CanLII links)
    Wilson and Boswell, supra
  5. Sekhon, supra at para 49


The necessity requirement is not a strict standard. It is necessary where it furnishes scientific information that is likely to be outside the experience and knowledge of the trier of fact.[1]

The expert evidence must likely outside the ordinary experience and knowledge of the trier of fact.[2] The evidence should not be necessary if “on the proven facts a judge or jury can form their own conclusions without help, then the opinion of [an] expert is unnecessary”.[3]

However, the purpose of this element is to "ensure that the dangers associated with expert evidence are not lightly tolerated" and that mere "helpfulness" is not sufficient.[4]

The subject-matter should be one that a lay person is "unlikely to form a correct judgment about it, if unassisted by persons with special knowledge."[5]

The purpose of the necessity requirement it to protect against the dangers of expert evidence, including the risk of usurping the trier-of-fact, the consumption of time, and consumption of expenses.[6]

  1. R v B(RH), [1994] 2 SCR 656, 1994 CanLII 127
  2. R v Burns, 1994 CanLII 127 (SCC), [1994] 1 SCR 656 at paras 24-25
    R v Mohan, 1994 CanLII 80 (SCC), [1994] 2 SCR 9, (1994), 89 CCC (3d) 402 (S.C.C.) at pp. 414-415
  3. Mohan, ibid. at p. 23
    R v Sekhon, 2014 SCC 15 (CanLII) at para 45
  4. Abbott, supra at para 21
  5. R v Kelliher (Village of) v Smith, [1931] SCR 672, 1931 CanLII 1 (SCC)
    R v DD, [2000] 2 SCR 275, 2000 SCC 43 (CanLII), at para 57
  6. Mohan, supra at p. 24
    Sekhon, supra at para 45
    R v D.D., 2000 SCC 43 (CanLII), [2000] 2 SCR 275

Properly Qualified Expert

The burden is upon the party calling the witness to prove on a balance of probabilities that they are a "properly qualified expert".[1] The determination of whether a witness is "properly qualified" is based on a "case-specific determination".[2]

  1. R v Terceira, 1998 CanLII 2174 (ON CA) aff'd at 142 CCC (3d) 95 (SCC)
  2. R v Vander Wier, 2013 ONSC 7390 (CanLII), para 13


Factors to consider in the evaluation of whether the witness is "properly qualified" include:[1]

  • The manner in which the witness acquired the special skill and knowledge upon which the application is based;
  • The witness' formal education (i.e. degrees or certificates);
  • The witness' professional qualifications (i.e. a member of the College of Physicians and Surgeons);
  • The witness' membership and participation in professional associations related to his or her proposed evidence;
  • Whether the witness has attended additional courses or seminars related to the areas of evidence in dispute;
  • The witness' experience in the proposed area(s);
  • Whether the witness has taught or written in the proposed area(s);
  • Whether, after achieving a level of expertise, the witness has kept up with the literature in the field;
  • Whether the witness has previously been qualified to give evidence in the proposed area(s), including the number of times and whether the previous evidence was contested;
  • Whether the witness has not been qualified to give evidence in the proposed area(s) and if so, the reason(s) why; and
  • Whether previous caselaw or legal texts have identified the contested area as a proper area for expert evidence and if so, who might give the evidence.
  1. R v Pham, 2013 ONSC 4903 (CanLII), Durno J. at para. 31


A person cannot become a reliable expert by simply conducting "numerous casual conversations with drug users or traffickers". Mere anecdotal evidence cannot be tested or verified.[1]

  1. R v Klassen, 2003 MBQB 253 (CanLII), [2003] M.J. No. 417 at para 26 - drug expert rejected

Experience and Education

An expert does not have to have practical experience.[1]

The essential requirement is that "he should be skilled ... that which he is called upon to give an opinion."[2]

No Prior Qualification
There is no special rule for "first time" experts that would render them any less eligible for qualification as an expert.[3]

A formal education such as a university degree is not a necessary requirement to be an expert in an area.[4]

However, the expertise must be gained through some form of study or experience.[5] Deficiencies in expertise will usually go to weight not qualifications.[6]

Employment by Police Service
There is no impediment to qualification where the proposed expert is employed by police service.[7]

  1. e.g. R v Morgentaler (No. 2) (1973), 14 CCC (2d) 450 (Que. S.C.)(*no CanLII links) - gynecologist testified as expert despite having not practiced for 10 years
  2. R v Korski, 2007 MBQB 184 (CanLII), at para 15 citing McWilliams' Canadian Criminal Evidence
  3. R v Plourde, 2017 ABCA 367 (CanLII), at para 5
  4. R v Dugandzic [1981] O.J. No. 1, 57 CCC (2d) 517 (Ont. C.A.)(*no CanLII links) - chemistry degree not required to identify a still
  5. Dugandzic, supra
    R v Godfrey, 1974 ALTASCAD 43 (CanLII), (1974) 18 CCC (2d) 90 (Alta. S.C.A.D.) at pp. 102-104
  6. R v Marquard, 1993 CanLII 37 (SCC), [1993] 4 SCR 223 at p. 243
  7. Plourde, supra, at para 5

Independence and Impartiality

A qualified expert witness has as duty to the court to be "fair, objective and non-partisan".[1] An expert unable or unwilling to fulfill this duty "is not qualified to give expert evidence" and should not be allowed to do so.[2] The duty arises out of the obligations for the expert to be impartial, independent, and without bias.[3]

Independence is a factor in the fourth part of the Mohan test when balancing risks against the benefit of the evidence.[4]

The appropriate standard of a qualified expert is one that "would not change regardless of which party retained him or her".[5]

Disqualification for lack of impartiality is a high threshold and should be a "rare" occurrence.[6]

The appearance of bias on the part of an expert can render his opinion evidence inadmissible.[7]

Effect of Failing to Meet the Standard
A lack of independence has traditionally not been a prerequisite to admissibility. Rather partiality usually goes to weight[8] Some degree of favouritism is expected.[9] Where the expert evidence is not independent, the weight should correspond to "the degree to which their opinions are supported or contradicted by other evidence and common sense". With "little or no support" the evidence could be rejected.[10]

The lack of impartiality or ability to fulfill their primary duty to the court should render their opinion evidence inadmissible due to lack of impartiality or independence.[11]

There is suggestion that the expert's opinion may be inadmissible where the lack of independence creates enough prejudice to warrant exclusion.[12]

Role of Trial Judge as Gatekeeper
The trial judge must consider this duty at the "gatekeeper" stage of admission and may affect admission and weight.[13]

In certain circumstances, a voir dire has been required to test the alleged lack of independence against the opinion evidence, in particular the assumptions, known facts, and the level of expertise.[14] But it should not be "advocacy dressed up as expert opinion".[15]

Qualities of an Independent Witness
Qualities of an independent witness should include:[16]

  • expert opinion should be and should be seen to be the independent product of the expert uninfluenced as to form or content by the exigencies of litigation;
  • the expert witness should provide independent assistance to the Court by way of objective unbiased opinion in relation to matters within his expertise. The expert witness should never assume the role of advocate.
  • An expert witness should state the facts or assumptions on which his opinion is based. He should not omit to consider material facts which detract from his concluded opinion
  • An expert witness should make it clear when a particular question or issue falls outside his expertise.
  • If an expert's opinion is not properly researched because he considers that insufficient data is available then this must be stated with an indication that the opinion is no more than a provisional one
  • If after exchange of reports, an expert witness changes his view on a material matter ... such change of view should be communicated ... to the other side without delay and when appropriate to the Court.
  • Where expert evidence refers to photographs, plans, calculations ... survey reports or other similar documents [these] must be provided to the opposite party at the same time as the exchange of reports

A retired police officer, experienced and training in investigating organized biker gangs is acceptable evidence.[17]

Courts are often concerned with impartiality where the expert was involved in the application for a search warrant or the laying of charges.[18]

Evidence Outside of Expertise Undermines Independence
Impartiality requires that the expert stay within the limits of their expertise and qualify their statements.[19] When stating opinion they should be clear to distinguish their own professional opinion from "more generally accepted scientific knowledge" when they diverge.[20]

Burden of Proof
The burden is upon the party challenging qualification on the basis of bias to prove that there is a "realistic concern" that the witness is unwilling or unable to comply with the duty to provide "fair, non-partisan, and objective assistance".[21]

  1. Burgess Langille Inman v Abbott and Haliburton Co, 2015 SCC 23 (CanLII), per Cromwell J at para 2
  2. Abbott, ibid. at para 2
  3. Abbott, ibid. at para 32 ("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.")
  4. Abbott, supra at para 53 to 54
    McManus, supra at para 66
  5. White Burgess v Abbott, at para 32
  6. Abbott, supra at para 49
  7. R v Van Bree, 2011 ONSC 4273 (CanLII)
    R v Kovats, 2000 BCPC 176 (CanLII)
  8. R v Payette, 2010 MBQB 73 (CanLII) at paras 16 to 18 R v Violette, 2008 BCSC 920 (CanLII) at para 106 R v Klassen, 2003 MBQB 253 (CanLII) at para 33
  9. Violette, supra, at para 101
  10. Klassen, supra, at para 32
  11. White Burgess v Abbott, at para 35
  12. e.g. Payette, supra
    Abbott, supra at para 37
  13. Abbott, supra at para 45
    R v McManus, 2017 ONCA 188 (CanLII) at para 65
  14. R v INCO Ltd., 2006 CanLII 14962 (ON SC), (2006), 80 O.R. (3d) 594 (Ont. Sup. Ct.)
  15. Fraser River Pile & Dredge Ltd. v Empire Tug Boats Ltd. (1995), 37 C.P.C. (3d) 119, at p. 126
  16. Payette, supra at para 21
  17. Violette, supra
  18. R v Snowdon, 2016 NSSC 321 (CanLII) at para 25
  19. R v Olscamp, 1994 CanLII 7553 (ON SC) at para 24 and 29
  20. Olscamp, ibid. at para 24
  21. R v McManus, 2017 ONCA 188 (CanLII) at paras 56 to 75

Evaluating Novel Fields

Novel sciences must be considered on a case-by-case basis. There are no fixed categories where the requirements for expert evidence have been met.[1] The courts must individually distinguish between new sciences and "junk" sciences. As such, needs "special scrutiny". [2]

Novel sciences must only be qualified if it is (1) necessary and (2) reliable[3]

When evaluating scientific evidence, the Court should consider Daubert factors:[4]

  1. falsifiability of the theory
  2. peer review and publication of the theory
  3. known or potential rate of error and the existence of standards controlling the research on which the theory is based and
  4. general acceptance of the methodology underlying the theory in the scientific community.

Unsettled Theories
The evidence can still be admissible even if the scientific theory is open to debate or that there are exceptional cases to the theory.[5]

It is not necessary to establish that the field be one that is subject to "peer review".[6]

The fact that an area of scientific theory is open to debate and that exceptional cases fall outside the norm does not preclude the evidence from admissibility.[7]

Source of Expertise
Expertise "may be acquired through study".[8]

Depending on the subject, it may not be necessary that the expert have a university degree in the subject to be an expert.[9]

The absence of writing or publications will generally go to weight and not admissibility.[10]

An expert is entitled to develop that expertise by observations, by talking to other experts, and by general involvement in the field.[11]

  1. e.g. R v Lavallee, 1990 CanLII 95 (SCC), [1990] 1 SCR 852 battered wife syndrome accepted as science
  2. R v J.(-L.), 2000 SCC 51 (CanLII)
  3. R v Terceira (1998) 123 CCC 3d 1 (ONCA),1998 CanLII 2174
  4. R v J-LJ, 2000 SCC 51 (CanLII), [2000] 2 SCR 600
  5. R v M.(B.), 1998 130 CCC 353 (ONCA), 1998 CanLII 13326 (ONCA)
  6. R v Abbey (2009), 246 CCC (3d) 301, [2009] O.J. No. 3534, 2009 ONCA 624 (ONCA) at para 97 - lower court erred in requiring peer review
  7. R v BM, 1998 CanLII 13326 (ON CA) ("The fact that a scientific theory is open to debate, however, or that exceptional cases fall outside the norm, does not preclude the admissibility of opinion evidence based on that theory.")
  8. McWilliams’ Canadian Criminal Evidence (4th) at 12:30.20.50. citing Mohan at 414
  9. R v Bulman, 2007 ONCA 169 (CanLII), [2007] O.J. No. 913 (QL) (C.A.) at para 7
  10. BM, supra at para 71
  11. R v Plourde, 2017 ABCA 367 (CanLII), at para 6 - ("An expert on drug activity is entitled to develop that expertise by observations of the drug trade, by talking to other experts, and by general involvement in policing of the drug trade.")