Expert Evidence

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General Principles

Expert opinion is testimonial evidence that gives an opinion on facts perceived by him or another that concerns an issue that is likely outside the experience and knowledge of the trier-of-fact (i.e. a lay person). [1] The expert must have special knowledge in the matter [2] and the opinion must be reasonably necessary to assist the trier-of-fact to make a proper judgment. [3] There must also be no exclusionary rules that would otherwise prohibit the evidence from being given. [4]

Specialized Knowledge Vs Expert Opinion

A person testifying to specialized knowledge will not necessarily be required to be qualified as an expert. Where they testify to their "factual knowledge" based on their "knowledge, observations and experience".[5]

It has been accepted that technical evidence describing the "general rule and its exceptions" of the functioning of complex systems is not opinion evidence where the "understand[ing] the scientific and technical underpinnings" are not necessary to give reliable descriptions.[6]

An expert who testifies to direct observation without opinion is not subject to the opinion rule of exclusion. This evidence is admitted in the same way as eye-witness evidence.[7]

The expert is not to testify to facts, but rather only opinion to allow the trier-of-fact to draw inferences. [8]


Relevance is "a threshold requirement" to admitting expert evidence.[9]


The key requirement for expert evidence is that the expert "possesses special knowledge and experience going beyond that of the trier of fact".[10] As long as the court is satisfied that the witness is sufficiently experienced he should be qualified.[11]

Any deficiencies of the knowledge or opinion simply goes to weight.[12]

The means by which the skill, expertise, or knowledge was acquired goes to weight and not admissibility.[13] It is acceptable to obtain the ability to give an opinion from study, instruction, practical experience or observations. Formal study in not necessary.[14]


An expert in any type of litigation has duties and responsibilities that include:[15]

  1. Expert evidence presented to the court 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.
  2. An expert witness should provide independent assistance to the court by way of objective unbiased opinion in relation to matters within his [or her] expertise. An expert witness . . . should never assume the role of an advocate.
  3. An expert witness should state the facts or assumptions upon which his [or her] opinion is based. He [or she] should not omit to consider material facts which could detract from his [or her] concluded opinion.
  4. An expert witness should make it clear when a particular question or issue falls outside his [or her] expertise.
  5. If an expert's opinion is not properly researched because he [or she] considers [there to be] insufficient data . . . available, then this must be stated with an indication that the opinion is no more than a provisional one. In cases where an expert witness, who has prepared a report could not assert that the report contained the truth, the whole truth and nothing but the truth without some qualification, that qualification should be stated in the report

The expert's right to give opinion evidence imports an obligation of maintaining an attitude of "strict independence and impartiality".[16]

It has been recommended that an "evidence-based approach" be taken to evaluate experts. This suggests four requirements:[17]

  1. the theory or technique utilized by the expert must be reliable, and used in a manner that is reliable;
  2. the expert must keep an open mind to a "broad menu of possibilities" (not be biased);
  3. the expert must be objective and comprehensive in collecting evidence -- including rejecting information not germane and transparent about the information and influences involved; and
  4. the expert must proffer more than the mere opinion, including the complete reasoning process, shortcomings and fair guidance on the confidence in the opinion.
Jury Instruction

Wher qualified expert gives opinion evidence outside of their field, the problem can generally be remedied through a "remedial instruction advising the jury to disabuse their minds of the inadmissible evidence".[18]

Standard of Review

Trial judges are entitled to "wide discretion" and deference when weighing the costs and benefits of admitting expert evidence.[19]

Whether someone can be qualified as an expert is a question of law and is reviewable on a standard of correctness.[20]

  1. Folkes v Chadd (1782), 3 Dougl. 157
    R v J-L J, [2000] 2 SCR 600, 2000 SCC 51 (CanLII), per Binnie J, at para 56
    R v Abbey, 1982 CanLII 25 (SCC), [1982] 2 SCR 24, per Dickson J
    R v Mohan, 1994 CanLII 80 (SCC), [1994] 2 SCR 9, per Sopinka J at 413 (the opinion must "provide information ‘which is likely to be outside the experience or knowledge of’” the trier of fact")
    R v Bunniss (1964), 44 CR 262 (BC Co. Ct.), 1964 CanLII 673 (BC SC), per Tyrwhitt-Drake J (an expert is "one who has by dint of training and practice, acquired a good knowledge of the science or art concerning which his opinion is sought, and the practical ability to use his judgment in that science")
  2. R v Terceira, 1998 CanLII 2174 (ON CA), (1998) 123 CCC 1 (Ont. CA), per Finlayson JA Mohan, supra, at p. 414 (the witness must be “shown to have acquired special or peculiar knowledge through study or experience in respect of the matters on which he or she undertakes to testify”)
  3. Mohan, supra
    R v Lovie, 1995 CanLII 801 (ON CA), per Finlayson JA
  4. R v NO, 2009 ABCA 75 (CanLII), per curiam, at para 19
  5. R v Hamilton, 2011 ONCA 399 (CanLII), per curiam, at paras 273 to 284 - evidence from phone company as to the mechanical workings of cell towers and their relationship to the cell phone
    R v Ranger, 2010 ONCA 759 (CanLII), per curiam -- cell phone tower evidence
    cf. R v Korski, 2009 MBCA 37 (CanLII), per Steel JA -- required expert to testify on cell tower evidence
  6. Hamilton, supra, at paras 273, 274, 277
  7. R v KA, 1999 CanLII 3793 (ON CA), per Charron JA, at para 72
  8. R v Parrott, 2001 SCC 3 (CanLII), [2001] 1 SCR 178, per Binnie J
    R v Lavallee, 1990 CanLII 95 (SCC), [1990] 1 SCR 852, per Wilson J
  9. Mohan, supra, at p. 411
  10. R v Béland, 1987 CanLII 27 (SCC), [1987] 2 SCR 398, per McIntyre J, at p. 415
    R v Marquard, 1993 CanLII 37 (SCC), [1993] 4 SCR 223, per McLachlin J, at para 35
    R v Chan, 1993 ABCA 383 (CanLII), (1993), 145 A.R. 304, per curiam, at para 9
  11. Chan, ibid., at para 9
  12. Chan, ibid., at para 9
  13. Chan, ibid., at para 9 citing Sopinka, Lederman and Bryant, The Law of Evidence of Canada (1992), at pp. 536‑537: ("As long as the court is satisfied that the witness is sufficiently experienced in the subject matter at issue, the court will not be concerned with whether his or her skill was derived from specific studies or by practical training, although that may affect the weight to be given to the evidence.") R v Rayner, 2000 NSCA 143 (CanLII), 189 N.S.R. (2d) 144, per Saunders JA
  14. R v Melaragni, (1992), 73 CCC (3d) 348 (Ont. Ct. Gen. Div.), 1992 CanLII 12764 (ON SC), per Moldaver JA
    R v NO, 2009 ABCA 75 (CanLII), per curiam, at para 22
  15. Bedford v Canada, 2010 ONSC 4264 (CanLII), per Himel J, at para 100 - upheld on appeal 2013 SCC 72
    see also National Justice Compania Naviera SA v Prudential Assurance Co. (The "Ikarian Reefer"), [1993] 2 Lloyd's Rep. 68 (Q.B. (Comm. Ct.)), at pp. 81-82
  16. Bedford, supra, at para 101
  17. see Bedford, supra, at para 102 - referencing the Goudge Inquiry
    Professor David Paciocco in "Taking a 'Goudge' out of Bluster and Blarney: an 'Evidence-Based Approach' to Expert Testimony" (2009), 13 Can. Crim. L.R. 135
  18. R v Sekhon, 2014 SCC 15 (CanLII), per Moldaver J, at para 48
  19. R v Clark, 2016 ABCA 72 (CanLII), per curiam, at paras 59 to 62
    R v DD, 2000 SCC 43 (CanLII), per Major J, at para 13
    R v Soni, 2016 ABCA 231 (CanLII), per curiam (2:1), at para 8
    R v Dominic, 2016 ABCA 114 (CanLII), per curiam, at para 17
    R v Abbey, 2009 ONCA 624 (CanLII), per Doherty JA, at para 97, leave to appeal to SCC refused
  20. R v Bear (C.W.), 2013 MBCA 96 (CanLII), per Steel JA, at para 81

Requirements of Expert Evidence ("Mohan" Test)



Once the witness is qualified as an expert they are permitted to give evidence in the area for which they were qualified.

Requirement of Citing Sources

The expert must cite the bases on his opinion in order for the trier-of-fact to evaluate the opinion. [1] The expert can use sources and information found in the "scope of his or her expertise"[2] as well as sources outside his knowledge in limited contexts.[3] The "second hand" evidence of texts is not admissible as evidence, but can be used to show the information on which the opinion is based.[4]

Questions on Reports, Texts, and Articles

An expert may be cross-examined on a text, report or article relating to his area of expertise. [5] The expert can only comment on works they are familiar with. If the expert acknowledges the authority, the examiner may read parts of the document to the witness to the extent that it is accepted as valid. Those confirmed passages will become evidence in the case[6]

Rejecting Uncontradicted Expert Evidence

A judge may reject the uncontradicted expert evidence as unreasonable. [7] The evidence should not be rejected if there is no contradictory evidence and the opinion is not seriously challenged.[8]

Statistical and Annecdotal Evidence

The expert cannot recite statistical evidence of probabilities based on prior similar events to infer what likely occurred in the incident at issue.[9]

The expert may not give anecdotal evidence concerning prior similar events to suggest an opinion about the event at issue.[10]

Area of Expertise

The qualified "area of expertise" does not need to articulate the exact subject matter to which they will give evidence. It often sufficient to articulate the general area of knowledge.[11]

Use of Report

It is common-place that an expert may testify with their reports on hand and may refer to them during testimony.[12]

There is some division of whether an expert report should be filed as an exhibit.[13]

  1. R v Neil, 1957 CanLII 70 (SCC), [1957] SCR 685
    R v Grandinetti, 2003 ABCA 307 (CanLII), per McFadyen JA (2:1)
    R v Trudel, 1994 CanLII 5397 (QC CA), per Brossard JA
  2. R v SAB, 2003 SCC 60 (CanLII), [2003] 2 SCR 678, per Arbour J, at para 63
  3. R v Anderson (1914), 22 CCC 455, 1914 CanLII 361 (AB CA), per Harvey J
    R v Godfrey, 1974 ALTASCAD 43 (CanLII), [1974] 4 W.W.R. 677, 18 CCC (2d) 90 (Alta. C.A.) (2:1), at pp. 102-104
  4. R v Burns, 1994 CanLII 127 (SCC), [1994] 1 SCR 656, per McLachlin J
  5. R c Taillefer, 1995 CanLII 4592 (QCCA), per Proulx JA
    R v Marquard, 1993 CanLII 37 (SCC), [1993] 4 SCR 223, per McLachlin J
  6. Marquard, ibid. (" examining an expert witness on other expert opinions found in papers or books is to ask the witness if he or she knows the work. If the answer is "no", or if the witness denies the work's authority, that is the end of the matter. ...If the answer is "yes", and the witness acknowledges the work's authority, then the witness has confirmed it by the witness's own testimony. Parts of it may be read to the witness, and to the extent they are confirmed, they become evidence in the case.")
  7. R v Prince, (1971), 6 CCC (2d) 183 (Ont. C.A.), 1971 CanLII 1285 (ON CA), per Gale CJ
    R v Lambkin, 2002 MBCA 157 (CanLII), per Monnin JA
  8. R v Molodowic, 2000 SCC 16 (CanLII), [2000] 1 SCR 420, per Arbour J
  9. R v Klymchuk (2005), 2005 CanLII 44167 (ON CA), 203 CCC (3d) 341 (Ont. C.A.), per Doherty JA, at para 46
    R v Shafia, 2016 ONCA 812 (CanLII), per Watt JA, at para 242 ("An expert is not entitled to give statistical evidence of probabilities based on prior similar events to support a conclusion about what happened on the occasion that forms the subject-matter of charges")
  10. Shafia, ibid., at para 243 ("Experts may not give anecdotal evidence gathered from prior experiences in proffering their opinion about conduct on a particular occasion. The evidence lacks legal relevance and is apt to engender significant prejudice, especially when adduced to rebut a defence")
    Sekhon, supra, at paras 49 to 50
  11. e.g. R v Rothgordt, 2017 BCCA 230 (CanLII), per Frankel JA, at para 16
  12. R v Sandham, [2009] OJ No 4517 (Ont. S.C.J.), 2009 CanLII 58982 (ON SC), per Heeney J ("Experts are routinely permitted to have their reports in front of them as they testify, and to refer to them as they deliver their evidence. Such reports are frequently entered as exhibits.")
  13. R v Millard and Smich, 2016 ONSC 1517 (CanLII), per Goodman J (" Generally speaking, I agree that experts’ reports, per se, ought not to be filed as exhibits. The evidence is their “in-court” testimony. ")

Weight of Opinion

Expert opinion on anything that is not necessary for the trier-of-fact to make findings is not admissible.[1]

The opinion evidence given must be within the qualified field of expertise.[2]

The expert need not have personal knowledge of any facts to give an opinion.[3]

A jury must be instructed that the hearsay evidence of the facts underlying the experts opinion is not admissible for any purpose other than to evaluate the weight of the opinion.[4]

The weight given to an opinion is a question of fact.[5]

The opinion must be specific to the case and not simply in generalities.[6]

Requirements for a Factual Foundation

Before a judge can rely upon an expert opinion, the expert must give evidence on the factual foundation relied upon to form the opinion.[7] Where the underlying facts are not established in evidence, the judge cannot rely upon the opinion.[8]

The evidence relied upon for the opinion can include second-hand evidence, but that the it may affect the weight accorded to the opinion.[9]

Evaluating Conflicting Expert Evidence

Where the evidence of multiple experts conflict, it is not a matter of chosing one expert over the other, but rather giving weight to each expert and review the opinions as a whole.[10]

Evaluating Expert Evidence Without A Conflicting Evidence

A judge need not believe or make any findings of fact based on the testimony of an expert witness due to the failure of the other side to adduce contradicting evidence.[11]

  1. R v Howard, 1989 CanLII 99 (SCC), [1989] 1 SCR 1337, per Lamer J
    R v Béland, 1987 CanLII 27 (SCC), [1987] 2 SCR 398, per McIntyre J R v Millar, (1989), 49 CCC (3d) 193 (ONCA), 1989 CanLII 7151 (ON CA), per Morden JA, at 220
  2. Howard, supra
    Millar, supra
  3. R v Preeper and Doyle, 1888 CanLII 56 (SCC), (1888), 15 SCR 401
  4. R v Babcock, 1984 ABCA 291 (CanLII), per Moir JA
  5. R v Smithers, 1977 CanLII 7 (SCC), [1978] 1 SCR 506, per Dickson J
  6. R v Li, 1980 CanLII 344 (BC SC), per Trainor J
  7. R v Neil, 1957 CanLII 70 (SCC), [1957] SCR 685
    R v Dietrich, 1970 CanLII 377 (ON CA), per Gale CJ
  8. R v Abbey, 1982 CanLII 25 (SCC), [1982] 2 SCR 24, per Dickson J ("Before any weight can be given to an expert's opinion, the facts upon which the opinion is based must be found to exist.")
    R v Morgentaler (No. 2) (1973), 14 CCC (2d) 450 (Que. S.C.), 1973 CanLII 1462 (QC CQ), per Hugessen ACJ R v Lupien, [1970] SCR 263, 1969 CanLII 55 (SCC)
    Howard, supra
    R v Phillion, 1977 CanLII 23 (SCC), [1978] 1 SCR 18, per Ritchie J
  9. R v Alcantara, 2012 ABQB 225 (CanLII), per Greckol J, at para 125
  10. R v Jonkman, 2012 SKQB 511 (CanLII), per Schwann J, at para 97
    Toneguzza-Norvell v Burnaby Hospital, 1994 CanLII 106 (SCC), [1994] 1 SCR 114, per McLachlin J
    Housen v Nikulaison, 2002 SCC 33 (CanLII), [2002] 2 SCR 235, per Iacobucci and Major JJ
  11. R v Doodnaught, 2017 ONCA 781 (CanLII), per Watt JA, at para 124 (" with the testimony of any witness, a trial judge need not believe or make findings of fact based on the testimony of an expert witness simply because no witness is called by the opposite party or evidence adduced to contradict it")
    R v Moke (1917), 1917 CanLII 426 (AB CA), 28 CCC 296 (Alta. S.C., A.D.), per Walsh J, at p. 300

Hypothetical Questions

The opinion evidence can be related by way of hypothetical questions put to the expert.[1] However, the testimony cannot be "oath-helping" evidence.[2]

The hypotheticals can be put to the expert where the facts are not in dispute.[3] If the underlying facts are in dispute the opinion cannot remove the fact-finding function of the trier-of-fact.[4] The cross-examiner can put to the expert any proven or provable facts to determine whether it alters the confidence of the opinion.[5]

It can be preferrable to ask questions in hypothetical form on contradictory matters in order to avoid overly influencing the jury's fact-finding process.[6]

The party seeking to rely on an expert opinion based on a hypothetical fact has the burden of establishing those underlying facts.[7]

  1. R v Fiqia, 1994 ABCA 402 (CanLII), (1994) 162 A.R. 117 (C.A.), per Hunt JA
  2. R v Reid, 2003 CanLII 14779 (ON C.A.), per Moldaver JA
  3. R v Bleta, 1964 CanLII 14 (SCC), [1964] SCR 561, per Ritchie J
  4. R v PG, 2009 ONCA 32 (CanLII), per Juriansz JA
  5. R v Kerr, 2000 BCCA 209 (CanLII), per McEachern CJ
  6. R v Leinen, 2013 ABCA 283 (CanLII), per Hunt JA (2:1)
  7. R v Lavallee, [1990] 1 SCR 852, 1990 CanLII 95 (SCC), per Wilson J
    R v Flight, 2014 ABCA 185 (CanLII), per Veldhuis JA

Ultimate Issue Rule

An expert should generally not give an opinion on an "ultimate issue" to the case otherwise the expert would usurp the role of the trier-of-fact.[1] This is not a strict rule as it should be determined on a case-by-case.[2] Where the facts are not in dispute there is a discretion for the judge to allow the expert to give evidence on the ultimate issue.[3]

A psychologist has been allowed to give evidence on whether a murder was "planned and deliberate".[4] Also whether the accused's condition is a "disease of the mind".[5]

  1. R v Béland, 1987 CanLII 27 (SCC), [1987] 2 SCR 398, per McIntyre J
  2. R v Potts, 2018 ONCA 294 (CanLII), per curiam, at para 47 ("It is worth recalling that no general rule precludes the introduction of expert opinion evidence on the ultimate issue in a criminal trial")
    R v Mohan, 1994 CanLII 80 (SCC), [1994] 2 SCR 9, per Sopinka J, at pp. 24-25
    R v Bryan (2003), 2003 CanLII 24337 (ON CA), 175 CCC (3d) 285 (Ont. C.A.), per Goudge JA, at paras 16-17
    R v Lucas, 2014 ONCA 561 (CanLII), 121 O.R. (3d) 303, per curiam, at para 271
  3. Swietlinski v R, 1978 CanLII 56 (ON CA), per Martin JA
  4. R v More, 1963 CanLII 79 (SCC), [1963] SCR 522, per Cartwright J
  5. R v Cooper, 1979 CanLII 63 (SCC), [1980] 1 SCR 1149, per Dickson J

Specific Types of Expert

Drug Expert

A lay person such as an RCMP officer cannot be a reliable expert simply by "conducting numerous casual conversations with drug users or traffickers." This sort of "[a]necdotal evidence cannot be tested or verified hearsay must be supplemented and supported by some form of admissible evidence."[1]

  1. R v Klassen, 2003 MBQB 253 (CanLII), [2003] M.J. No. 417, per Scurfield J

Example Fields of Qualifications

Opinion Evidence Outside of Qualification

Where a qualified expert testifies to matters directly outside of their area of qualification but is clearly within their area of special knowledge, the evidence can still be accepted absent any objection to the evidence by the opposing side.[1] Likewise, a technical failure to qualify someone who clearly has expertise in the area, absent any objections, should be permitted to give expert evidence.[2]

  1. R v Marquard, 1993 CanLII 37 (SCC), [1993] 4 SCR 223, per McLachlin J, at pp. 242-44
  2. Marquard, ibid., at pp. 242-44

Where Technical Evidence Does Not Require Expert Evidence

There is some difference in opinion of whether or not expert qualification is needed to present evidence from cell phone towers.[1] It is generally accepted that even where it is not necessary, it is useful to explain some of the records.[2]

Evidence on the regular functioning of social media websites can in circumstances be accepted without qualification.[3]

  1. R v Hamilton, 2011 ONCA 399 (CanLII), per curiam, at paras 273 to 284 - evidence from phone company as to the mechanical workings of cell towers and their relationship to the cell phone
    R v Ranger, 2010 ONCA 759 (CanLII), per curiam -- cell phone tower evidence accepted without qualification for general location evidence
    cf. R v Korski, 2009 MBCA 37 (CanLII), per Steel JA -- required expert to testify on cell tower evidence
  2. e.g. R v McBean, 2011 ONSC 3125 (CanLII), per Fuerst J
  3. R v Soh, 2014 NBQB 20 (CanLII), per LaVigne J

Case Digests

See Also