Our litigation bulletin series is designed to demystify the litigation process. To date, this year’s bulletins have focussed on types of proceedings and on specific rules of procedure. In this bulletin, we turn to the subject of evidence. Evidence is proof that is presented to a court or tribunal to persuade the judge, jury or tribunal members of the truth or falsity of a fact in issue in the proceedings.

At trials, evidence is introduced through live witnesses in a courtroom. Most witnesses testify about things they have seen, done or heard that are relevant to the proceeding. This type of witness is called a lay witness. Expert witnesses are a different category of witnesses. Experts provide courts with opinion evidence based on facts that they gather for the purpose of testifying in the proceeding. Expert testimony can be helpful in many cases, and is quite common in Canadian jurisdictions. This bulletin will identify basic matters that apply to expert evidence and offer practical considerations about using expert evidence.

What is Expert Evidence?

Lay witnesses possess firsthand knowledge of the facts to which they testify. Their testimony is usually limited to the facts within their knowledge or observation (as noted above, what they have seen, heard or done), and reasonable conclusions that they may draw from their sensory observations, rather than opinions. In contrast to lay witnesses, expert witnesses are not limited to testifying about their personal knowledge. An expert may offer opinions that go well beyond that which he or she has directly observed.

The opinion of an expert witness may be admitted into evidence to assist a judge or jury with technical or specialized evidence that allows the judge or jury to properly assess other evidence that is presented during the trial. An expert may also provide inferences which a judge or jury, because of the technical nature of the facts, would otherwise be unable to draw. For example, a chartered business valuator may be able to draw inferences about the value of a company based on a series of facts about that company and the valuator’s education in valuation principles. An engineering expert may be able to draw inferences about why a building collapsed because of his skill and expertise. A handwriting expert may be able to draw inferences about who wrote or did not write on a document based on her specialized knowledge.

Admissibility of Expert Evidence

Before a piece of evidence may be considered by a judge or jury in reaching a decision, the evidence must satisfy certain requirements. One of these requirements is that the evidence must be admissible. Evidence is admissible if it is relevant and is not subject to any exclusion under a rule of law. If evidence is not admissible, a court or judge will reject the evidence and will not consider it in arriving at a decision or making findings. The evidence will not form part of the court record on which the decision is based. Expert evidence will not be admissible unless it is relevant and necessary to assist the judge or jury. In addition, expert evidence may be excluded if the court to which the expert evidence is proffered does not find that the expert is properly qualified to provide expert evidence. Finally, expert evidence is not admissible if there is a rule of evidence that calls for its exclusion (an example of a common exclusionary rule with which many people may be familiar is the rule against hearsay).


As is the case with all evidence, expert evidence must be relevant to the matters at issue before it is admissible. Although relevant evidence is generally admissible, courts retain a discretionary power to exclude relevant evidence if the probative value of the evidence is outweighed by the potential prejudicial effect of evidence. Evidence may be prejudicial if, for example, it is misleading or distorts the fact-finding process.

Therefore, even if expert evidence is potentially relevant, a court may exclude it. This inquiry as to whether the prejudicial effect of evidence outweighs its probative value may be analogized to a “costbenefit” analysis. Does the value of the evidence outweigh any potential prejudicial effect it may have on the judge or jury? This question is very important because a judge or jury may be inclined to see expert evidence as flawless, and may therefore give such evidence more weight than it actually deserves. A court may exclude expert evidence if this concern about the prejudicial effect of the evidence outweighs the value that may be garnered from the expert evidence.


Expert evidence must be more than just helpful in order to meet the necessity threshold. The Supreme Court of Canada has found that expert evidence must be “necessary in the sense that it provide information which is likely to be outside the experience and knowledge of a judge or jury.” Expert evidence will generally only be admitted when lay persons are apt to come to the wrong conclusion without expert assistance, or where access to important information will be lost unless the judge or jury borrows from the learning of experts.

Properly Qualified

A judge must also decide whether a potential expert witness is properly qualified before the witness may give expert evidence. It is usually sufficient to show that the witness has special knowledge of the matters about which he or she intends to testify. This special knowledge is generally acquired though education, training or experience. Examples include a chartered accountant designation, a license to practice psychiatry, or experience working in a particular industry for many years.

The initial assessment by the court of an expert’s qualifications occurs when a potential expert witness first takes the witness stand in a courtroom, and before the judge agrees that the expert can testify about the matters at issue. The lawyer for the party who is tendering the expert as a witness can ask the expert questions about his or her qualifications to provide expert evidence, and the lawyer for any adverse party may cross-examine the expert about those qualifications.


A proposed expert witness must also be independent. If the expert is not independent of the party seeking to tender his or her evidence, the expert is not properly qualified. Experts cannot be seen to be advocates for the position of the party putting the expert forward as a witness. Similarly, it is not proper for legal counsel to attempt to persuade experts to alter their opinions to make them more favourable to their client. This does not mean, however, that lawyers cannot assist an expert to prepare for their testimony at trial by informing them of the questions to be asked during examination in chief and the potential questions to be asked during cross-examination.

The fact that an expert witness must be independent also does not mean that an expert may not be remunerated for preparing a report and testifying in court. However, an expert’s fee should be reasonable in all of the circumstances. Unreasonably high expert fees can place an expert witness in an awkward position during crossexamination. Expert testimony should be objective.

Once expert evidence is determined by the court to be admissible, legal counsel for the party calling the expert as a witness may elicit the opinions and theories of the expert in the most persuasive manner possible. Legal counsel for an adverse party may cross-examine an expert witness on their findings, opinions and conclusions in an attempt to persuade the judge or jury that the expert evidence should not be given much weight during their deliberations.

Expert Reports

Examinations for Discovery

A party on an examination for discovery may obtain disclosure of the findings, opinions and conclusions of an expert engaged by the party being examined, as well as the expert’s name and address. A party who retains an expert in a particular proceeding and decides not to call that expert as a witness at the trial of that proceeding does not have to disclose the findings, opinions and conclusions of that expert. Thus, parties can consult with experts for the purposes of litigation without the concern that opinions of their own expert that are adverse to their case will be used against them at trial.

When Is a Report Required?

The Ontario Rules of Civil Procedure govern when the testimony of an expert witness must be set out in a report and when an expert report must be served other parties. Generally, an expert witness cannot testify about any issue unless the substance of his or her testimony with respect to that issue is set out in a report prior to trial. In exceptional circumstances, judges may grant leave to permit an expert to testify without a report having been served.

A party who intends to call an expert witness at trial must serve that expert report on the other parties to the proceeding within the timeframe stipulated in the Rules of Civil Procedure. A party who intends to call an expert witness at trial to respond to the expert witness of another party must also serve that expert’s report within a specified timeframe. In some circumstances, courts will grant leave to abridge the time for service of expert reports, or will grant an adjournment of a trial date to provide a party with additional time to respond to an expert report.

The Expert’s File

Expert’s reports and their findings are prima facie protected by litigation privilege. They need not be disclosed as part of the documentary discovery process if the decision has been made not to call the expert witness at trial. When the decision is made to call the expert as a witness at trial, litigation privilege with respect to his final report is waived, and, as discussed above, the expert report must be served within certain time limits prior to trial.

Recent case law suggests that once a party decides to offer an expert’s opinion, the opposing party should be given access to the foundations of that opinion in order to be able to adequately test the validity of the opinion. Therefore, a party may be ordered to disclose communications made between an expert and the lawyer for the party who retained that expert. A party may also be required to disclose drafts of an expert’s final report. If a party is required to produce such drafts, the expert may be crossexamined on why changes were made between one version of a draft to another.

There are no hard and fast rules about what communications with experts must be disclosed and what need not be disclosed. This area of law is governed by case law, which is constantly evolving. Because expert testimony is frequently admitted into evidence at trials, litigation lawyers are generally familiar with how to retain experts, how to communicate with experts, the requirements for disclosure of such communications and the evolution of the law in this area.

Opinion on the Ultimate Issue

There is some debate in the case law about whether an expert may testify on the very issue that the court must decide, or the “ultimate issue”. Courts will be vigilant about enforcing the rules of relevance and necessity when an expert’s proposed testimony approaches an opinion on the ultimate issue. Courts may exclude expert evidence on the ultimate issue on the basis that the expert evidence is not necessary because the court is also capable of drawing the inference that the expert is drawing about the ultimate issue.

Number of Experts

There are rules in the Ontario Evidence Act and the Canada Evidence Act limiting the number of expert witnesses a party may call without leave (or permission) of a judge. These limits are an implicit acknowledgement that cases are not to be decided by a tally of the experts on each side of the case. Despite these limits, a court may grant leave to call additional experts where it is warranted, but parties should be respectful of the court’s time and should consider what value each expert’s testimony adds to the overall case. Courts will generally grant leave to call additional experts in order to avoid injustice, and where such a request is not seen as abusive or manifestly unnecessary.

Court-Appointed Experts

Under the Rules of Civil Procedure, a judge in Ontario is empowered to appoint one or more independent experts to inquire into and report on any question of fact or opinion relevant to an issue in the action. Such experts must be named by the judge and, where possible, should be an expert agreed on by the parties. Any court-appointed expert must prepare a report and all parties are entitled to a copy of that report. Any report that results from the appointment of the expert is properly part of the trial record, and counsel for the parties to the proceeding have a right to make submissions on such reports.

The practice of the court appointing an expert is rarely used. Parties to litigation who are represented by counsel will often receive advice from their lawyers about when it is necessary to retain an expert. Where parties have already retained properly qualified experts, it is usually unnecessary for the court to appoint an expert.


Expert evidence often plays a critical role in the litigation process. Like any key evidence, expert evidence can mean the difference between success and failure in a proceeding. Legal counsel should be consulted to provide parties with advice about the necessity and logistics of retaining experts to prepare reports and testify at trial.