Rules of Evidence for a Qualified Expert
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.  The expert can use sources and information found in the "scope of his or her expertise" as well as sources outside his knowledge in limited contexts. 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.
- Questions on Reports, Texts, and Articles
An expert may be cross-examined on a text, report or article relating to his area of expertise.  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
- Rejecting Uncontradicted Expert Evidence
- 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.
The expert may not give anecdotal evidence concerning prior similar events to suggest an opinion about the event at issue.
- 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.
- Use of Report
It is common-place that an expert may testify with their reports on hand and may refer to them during testimony.
There is some division of whether an expert report should be filed as an exhibit.
- Offer of Alternate Theories
A crown expert is permitted to offer up alternate theories of causation based on the evidence.
R v Neil, 1957 CanLII 70 (SCC),  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
- R v SAB, 2003 SCC 60 (CanLII), per Arbour J, at para 63
R v Anderson, 1914 CanLII 361 (AB CA), 22 CCC 455, per Harvey J
R v Godfrey, 1974 ALTASCAD 43 (CanLII),  4 WWR 677, 18 CCC (2d) 90 (2:1), at pp. 102-104
- R v Burns, 1994 CanLII 127 (SCC), per McLachlin J
R c Taillefer, 1995 CanLII 4592 (QCCA), per Proulx JA
R v Marquard, 1993 CanLII 37 (SCC), per McLachlin J
Marquard, ibid. ("...in 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.")
R v Prince, 1971 CanLII 1285 (ON CA), 6 CCC (2d) 183 (Ont. C.A.), per Gale CJ
R v Lambkin, 2002 MBCA 157 (CanLII), per Monnin JA
- R v Molodowic, 2000 SCC 16 (CanLII), per Arbour J
R v Klymchuk, 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")
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")
R v Sekhon, 2014 SCC 15 (CanLII), per Moldaver J, at paras 49 to 50
e.g. R v Rothgordt, 2017 BCCA 230 (CanLII), per Frankel JA, at para 16
R v Sandham, 2009 CanLII 58982 (ONSC),  OJ No 4517 (Ont. SCJ), 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.")
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. ")
- R v Biddersingh, 2020 ONCA 241 (CanLII), per Tulloch JA, at para 67
Proper Bounds of Opinion
It is the duty of the trial judge to ensure that the expert stays within the bounds of their expertise.
- Anecdotal Evidence
The recitation of "anecdotal" evidence about the expert's experience and what the expert has and has not seen carries with it a high degree of prejudice and may result in an error if relied upon for conviction. It is considered not legally relevant as guilt of persons that the expert "had encountered in the past is legally irrelevant to the guilt or innocence" of the accused. The trier of fact should not reason from a generalized ocnclusion based on prior experience to a specific state of mind of the person charged."
Anecdotal evidence is also often not necessary as individual experience may not be beyond the common knowledge and experience of a judge or a matter of technical or scientific in nature.
- R v Sekhon, 2014 SCC 15 (CanLII),  1 SCR 272, per Moldaver J, at para 46 ("...all trial judges ... have an ongoing duty to ensure that expert evidence remains within its proper scope."), 47 ("The trial judge must both ensure that an expert stays within the proper bounds of his or her expertise and that the content of the evidence itself is properly the subject of expert evidence.")
- R v Cook, 2020 ONCA 731 (CanLII), per Trotter JA, at para 100
R v Burnett, 2018 ONCA 790 (CanLII), 367 CCC (3d) 65, per Watt JA, at para 75
Cook, supra, at para 100
Cook, supra, at para 101
Sekhon, supra, at para 49
Weight of Opinion
Expert opinion on anything that is not necessary for the trier-of-fact to make findings is not admissible.
The opinion evidence given must be within the qualified field of expertise.
The expert need not have personal knowledge of any facts to give an opinion.
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.
The weight given to an opinion is a question of fact.
The opinion must be specific to the case and not simply in generalities.
- 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. Where the underlying facts are not established in evidence, the judge cannot rely upon the opinion.
The evidence relied upon for the opinion can include second-hand evidence, but that the it may affect the weight accorded to the opinion.
- Evaluating Conflicting Expert Evidence
Where the evidence of multiple experts conflict, it is not a matter of choosing one expert over the other, but rather giving weight to each expert and review the opinions as a whole.
- 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.
R v Howard, 1989 CanLII 99 (SCC), per Lamer J
R v Béland, 1987 CanLII 27 (SCC),  2 SCR 398, per McIntyre J R v Millar, 1989 CanLII 7151 (ON CA), 49 CCC (3d) 193 (ONCA), per Morden JA, at 220
- R v Preeper and Doyle, 1888 CanLII 56 (SCC), , 15 SCR 401
- R v Babcock, 1984 ABCA 291 (CanLII), per Moir JA
- R v Smithers, 1977 CanLII 7 (SCC), per Dickson J
- R v Li, 1980 CanLII 344 (BC SC), per Trainor J
R v Neil, 1957 CanLII 70 (SCC),  SCR 685
R v Dietrich, 1970 CanLII 377 (ON CA), per Gale CJ
R v Abbey, 1982 CanLII 25 (SCC), 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, 1973 CanLII 1462 (QC CQ), (No. 2) (1973), 14 CCC (2d) 450 (Que. S.C.), per Hugessen ACJ R v Lupien, 1969 CanLII 55 (SCC),  SCR 263
R v Phillion, 1977 CanLII 23 (SCC), per Ritchie J
R v Alcantara, 2012 ABQB 225 (CanLII), per Greckol J, at para 125
R v Jonkman, 2012 SKQB 511 (CanLII), per Schwann J, at para 97
Toneguzza-Norvell v Burnaby Hospital, 1994 CanLII 106 (SCC),  1 SCR 114, per McLachlin J
Housen v Nikulaison, 2002 SCC 33 (CanLII),  2 SCR 235, per Iacobucci and Major JJ
R v Doodnaught, 2017 ONCA 781 (CanLII), per Watt JA, at para 124 ("...as 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 CanLII 426 (AB CA), 28 CCC 296 (Alta. S.C., A.D.), per Walsh J, at p. 300
The hypotheticals can be put to the expert where the facts are not in dispute. If the underlying facts are in dispute the opinion cannot remove the fact-finding function of the trier-of-fact. The cross-examiner can put to the expert any proven or provable facts to determine whether it alters the confidence of the opinion.
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.
The party seeking to rely on an expert opinion based on a hypothetical fact has the burden of establishing those underlying facts.
- R v Fiqia, 1994 ABCA 402 (CanLII), , (1994) 162 A.R. 117 (CA), per Hunt JA
- R v Reid, 2003 CanLII 14779 (ONCA), per Moldaver JA
- R v Bleta, 1964 CanLII 14 (SCC),  SCR 561, per Ritchie J
- R v PG, 2009 ONCA 32 (CanLII), per Juriansz JA
- R v Kerr, 2000 BCCA 209 (CanLII), per McEachern CJ
- R v Leinen, 2013 ABCA 283 (CanLII), per Hunt JA (2:1)
R v Lavallee, 1990 CanLII 95 (SCC),  1 SCR 852 (CanLII), 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. This is not a strict rule as it should be determined on a case-by-case. 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.
- R v Béland, 1987 CanLII 27 (SCC),  2 SCR 398, per McIntyre J
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), per Sopinka J, at pp. 24-25
R v Bryan, 2003 CanLII 24337 (ON CA), 175 CCC (3d) 285 (Ont. C.A.), per Goudge JA, at paras 16 to 17
R v Lucas, 2014 ONCA 561 (CanLII), , 121 OR (3d) 303, per curiam, at para 271
- Swietlinski v R, 1978 CanLII 56 (ON CA), per Martin JA
- R v More, 1963 CanLII 79 (SCC),  SCR 522, per Cartwright J
- R v Cooper, 1979 CanLII 63 (SCC), per Dickson J
Change of Opinion
It is accepted that the testimony of witnesses, expert or otherwise, may change over time for various reasons. A mistrial would likely not be available where a witness gives advance notice that their evidence has changed.
- R v Chen, 2019 ONSC 3088 (CanLII), at para 22 ("Given this undeniable reality in criminal trials, it is difficult to imagine circumstances where it would be necessary to declare a mistrial simply because a witness gave advance notice to the lawyer who was planning on calling them, that they were going to, in fact, give unanticipated testimony at variance with some earlier indication of what their testimony might be.")
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."