Canadian Criminal Evidence/Opinion/Expert Evidence
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).  The expert must have special knowledge in the matter  and the opinion must be reasonably necessary to assist the trier-of-fact to make a proper judgment.  The expert is not to testify to facts, but rather only opinion to allow the trier-of-fact to draw inferences. 
Requirements to Adduced Expert Evidence
Section 657.3 permits a party to adduce expert evidence without the need to have the expert give viva voce evidence:
657.3 (1) In any proceedings, the evidence of a person as an expert may be given by means of a report accompanied by the affidavit or solemn declaration of the person, setting out, in particular, the qualifications of the person as an expert if
- (a) the court recognizes that person as an expert; and
- (b) the party intending to produce the report in evidence has, before the proceeding, given to the other party a copy of the affidavit or solemn declaration and the report and reasonable notice of the intention to produce it in evidence.
Attendance for examination
(2) Notwithstanding subsection (1), the court may require the person who appears to have signed an affidavit or solemn declaration referred to in that subsection to appear before it for examination or cross-examination in respect of the issue of proof of any of the statements contained in the affidavit or solemn declaration or report.
1997, c. 18, s. 80; 2002, c. 13, s. 62.
See also: s. 16 and s. 150.1 of the Code.
A party cannot call more than five witnesses without leave of the court.
Under 657.3, the judge may order compliance, seek particulars, or adjourn. He may not prohibit the witness from testifying who complies with the section.
Under s.7 of the Canada Evidence Act, where any party intends to "examine as witnesses, professional or other experts entitled according to the law or practice to give opinion evidence, not more than five of such witnesses may be called on either side without the leave of the court or judge or person presiding."
- s. 7 of the Canada Evidence Act
- R. v. Horan, 2008 ONCA 589 (CanLII)
A party may call a witness as an expert under s. 657.3 (1). A party intending to call the expert witness must give notice at least 30 days prior to the commencement of trial or within the time set by the judge. 
Section 657.3(3) states:
Notice for expert testimony
(3) For the purpose of promoting the fair, orderly and efficient presentation of the testimony of witnesses,
- (a) a party who intends to call a person as an expert witness shall, at least thirty days before the commencement of the trial or within any other period fixed by the justice or judge, give notice to the other party or parties of his or her intention to do so, accompanied by
- (i) the name of the proposed witness,
- (ii) a description of the area of expertise of the proposed witness that is sufficient to permit the other parties to inform themselves about that area of expertise, and
- (iii) a statement of the qualifications of the proposed witness as an expert;
- (b) in addition to complying with paragraph (a), a prosecutor who intends to call a person as an expert witness shall, within a reasonable period before trial, provide to the other party or parties
- (i) a copy of the report, if any, prepared by the proposed witness for the case, and
- (ii) if no report is prepared, a summary of the opinion anticipated to be given by the proposed witness and the grounds on which it is based; and
- (c) in addition to complying with paragraph (a), an accused, or his or her counsel, who intends to call a person as an expert witness shall, not later than the close of the case for the prosecution, provide to the other party or parties the material referred to in paragraph (b).
- s. 657.3(3)(a)
R. v. Salter, 2005 NSCA 129
If Notice is Not Given
Section 657.3(4) states that where the notice requirements of s. 657.3(3) are not complied with, the available remedies are as follows:
- "adjournment of the proceedings to the party who requests it to allow him or her to prepare for cross-examination of the expert witness;" (657.3(4)(a))
- an "order the party who called the expert witness to provide that other party and any other party with the material referred to in paragraph (3)(b);" and (657.3(4)(b))
- an "order the calling or recalling of any witness for the purpose of giving testimony on matters related to those raised in the expert witness’s testimony, unless the court considers it inappropriate to do so." (657.3(4)(c))
These remedies do not include the power to prohibit the party from calling the expert.
Other Use of the Materials
Under s. 657.3 (6), the prosecution is prohibited from using any of the materials given to him by defence for as part of its own case where the expert does not testify except for cross-examination without the consent of the accused. It further cannot be used for any other proceedings. (657.3(7))
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. The qualification process is one of delineating the boundaries of the evidence and the language used. 
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.
To qualify a person as an expert, the evidence must meet the Mohan requirements: 
- the opinion must be relevant;
- the opinion must be necessary to assist the trier-of-fact to draw the correct inference;
- the absence of any other exclusionary rule;
- the required qualifications of the proposed expert.
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). (R. v. K.(A.), 1999 137 CCC 3d 225 (ONCA))
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.(R. v. B.(R.H.),  2 SCR 656)
Qualified expert evidence can nonetheless be excluded if: (R. v. D.D.  2 SCR 275)
- the evidence would tend to usurp the duty of the trier of fact
- the prejudicial effect outweighs the probative value
- the time required outweighs its probative value
- the cost required outweighs the probative value
- 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, knowledge from "some" literature, and interviews. (R. v. Bedford (2000), 143 CCC 3d 311)
Expert evidence should be expected to be impartial to be admissible. There must not be any bias or appearance of bias.
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. The courts must individually distinguish between new sciences and "junk" sciences. As such, needs "special scrutiny". 
Novel sciences must only be qualified if it is (1) necessary and (2) reliable
The evidence can still be admissible even if the scientific theory is open to debate or that there are exceptional cases to the theory.
It is not necessary to establish that the field be one that is subject to "peer review".
Expertise "may be acquired through study".
Depending on the subject, it may not be necessary that the expert have a university degree in the subject to be an expert.
The absence of writing or publications will generally go to weight and not admissibility.
e.g. R. v. Lavallee,  1 S.C.R. 852 battered wife syndrome accepted as science
- R. v. J.(-L.), 2000 SCC 51
- R. v. Terceira (1998) 123 CCC 3d 1 (ONCA)1998 CanLII 2174
- R. v. M.(B.) 1998 130 CCC 353 (ONCA)1998 CanLII 13326
- R. v. Abbey (2009), 246 C.C.C. (3d) 301,  O.J. No. 3534, 2009 ONCA 624 (Ont. C.A.) at para. 97 - lower court erred in requiring peer review
- McWilliams’ Canadian Criminal Evidence (4th) at 12:30.20.50. citing Mohan at 414
- R. v. Bulman,  O.J. No. 913 (QL) (C.A.) at para. 7
R. v. B.M., 1998 CanLII 13326 (ON CA) at para. 71
Once the witness is qualified as an expert they are permitted to give evidence in the area for which they were qualified. The opinion evidence will be related by way of hypothetical questions put to the expert. However, the testimony cannot be "oath-helping" evidence.
The hypotheticals can be put to the expert where the facts are not 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 a the opinion.
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.
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.
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."
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.
- R. v. Fiqia (1994) 162 A.R. 117 (C.A.)
- R. v. Reid, 2003 CanLII 14779 (ON C.A.) 
- R. v. Bleta,  SCR 561 
- R. v. P.G., 2009 ONCA 32 
- R. v. Kerr, 2000 BCCA 209 
R. v. Neil,  S.C.R. 685 
R. v. Grandinetti, 2003 ABCA 307 
R. v. Trudel, 1994 CanLII 5397 (QC CA) 
- R. v. S.A.B., 2003 SCC 60,  2 S.C.R. 678 at 63 
R. v. Anderson (1914), 22 C.C.C. 455
R. v. Godfrey,  4 W.W.R. 677, 18 C.C.C. (2d) 90 (Alta. C.A.), at pp. 102-104
- R. v. Burns,  1 S.C.R. 656 
R. c. Taillefer, 1995 CanLII 4592 (QC C.A.) 
R. v. Marquard  3 SCR 223 
R. v. Conroy, (1993), Ont.C.A.
R. v. Prince, (1971) OntCA
R. v. Lambkin, 2002 MBCA 157 
- R. v. Molodowic, 2000 SCC 16,  1 S.C.R. 420 
- R. v. Klassen,  M.J. No. 417
R v Jonkman, 2012 SKQB 511 (CanLII) at para. 97
Toneguzza-Norvell v. Burnaby Hospital, 1994 CanLII 106 (SCC),  1 S.C.R. 114
Housen v. Nikulaison, 2002 SCC 33 (CanLII), 2002 SCC 33,  2 S.C.R. 235
Example Fields of Qualifications