Rules of Evidence for a Qualified Expert

This page was last substantively updated or reviewed January 2021. (Rev. # 96803)

• < Evidence
• < Opinion Evidence

General Principles

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 basis of 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 "secondhand" 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

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

Statistical and Anecdotal Evidence

The expert cannot recite statistical evidence of probabilities based on similar events prior to the incident 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 is often sufficient to articulate the general area of knowledge.

Use of Report

It is commonplace for an expert to testify with their reports on hand and 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.

Proper Bounds of Opinion

It is the duty of the trial judge to ensure that the expert stays within the bounds of their expertise.

The "opinion rule" does not prevent an expert from giving evidence of fact. The expert may express any firsthand observations that are relevant.

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. While it may be logically relevant, it is not legally relevant whether the expert had encountered other accused in the past who were guilty of an offence. The trier of fact should not reason from a generalized conclusion 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.

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 does not need to have personal knowledge of any facts to give an 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.

Hearsay

Where a factual premise of an expert is established solely by an out-of-court statement, the opinion can be entitled to less weight and sometimes no weight at all.

A jury must be instructed that the hearsay evidence of the facts underlying the expert's opinion is not admissible for any purpose other than to evaluate the weight of the opinion.

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 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 reviewing 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.

Hypothetical and Generic Questions

The opinion evidence can 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 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 preferable to ask questions in hypothetical form on contradictory matters in order to avoid overly influencing the jury's fact-finding process. Where hypotheticals are not used in contradicting an expert, "careful instructions" are required.

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

Generic questions

Generic evidence about practices can be necessary to assist the jury.

Ultimate Issue Rule

Traditionally, an expert should not give an opinion on an "ultimate issue" to the case; otherwise, the expert would usurp the role of the trier-of-fact. 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.

In modern application, this rule has largely disappeared.

However, the trial judge must apply "increased scrutiny" of the relevance and necessity when the evidence approaches the ultimate issue.

Example

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

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.

Specific Types of Expert

Drug Expert

A layperson 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."

Financial Expert

In a financial crime case, it can be appropriate for the Crown to lead "generic" evidence as to "legitimate and illegitimate ... banking practices and documents."