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Providing expert evidence is an essential part of the legal system, Judges and Juries place significant weight on your opinion. This article outlines the legal duties of an expert and suggests some strategies for making the best presentation in court.


The purpose of a trial is to allow a trier of fact (judge or jury) to hear and see “evidence” from which to draw conclusions and resolve a dispute between parties. In the evidence-gathering phase of a trial, the judge’s role is to act as a gatekeeper and only allow evidence which is relevant and necessary to determine the dispute by the trier of fact.  The vast majority of evidence comes from ordinary witnesses.  These witnesses tell or show the trier of fact some piece of relevant information that their senses have perceived.

The trier of fact may also hear evidence from an expert witness. The nature of expert evidence is different. Expert evidence is the statement of an opinion.

Judges have recognized that there are areas in which a trier of fact does not have the specialized knowledge, skill or experience to draw the correct inference from the facts presented.  A fair trial could not be conducted without a properly trained and experienced individual helping the trier of fact see how the facts ought to be interpreted. However, the concern most judges have is that the witness delivering the opinion may not present a reliable opinion or worse, may present a biased opinion.  If the trier of fact does not have the requisite knowledge, skill or experience to properly interpret the facts presented then how could this same trier of fact determine if they are being presented with a reliable opinion?

That, in a nut shell, is what we will speak about today.  How can you help a trier of fact understand that your opinion is reliable?

The Rules of Civil Procedure set out the requirements to bring a matter from inception to trial.  There are Rules which specifically apply to potential expert witnesses.  If you do not follow the Rules, it is most likely that you will never be accepted as an expert witness.

Duty of the Expert

The Duty of Expert rule (Rule 4.1) aims to clarify the role and duties of a potential expert witness. Rule 4.1 establishes that it is the duty of any potential expert to assist the court on matters within their expertise. This duty overrides any obligation to the party by whom, or on whose behalf, the expert is retained. Implicitly, the Rule is intended to correct the observation that experts are too often seen by the Court as advocates for the party who retained them or, as colloquially known “hired guns”.

Experts are to provide opinion evidence that is fair, objective and non-partisan:

4.1.01 (1) It is the duty of every expert engaged by or on behalf of a party to provide evidence in relation to a proceeding under these rules,

(a) to provide opinion evidence that is fair, objective and non-partisan;

(b) to provide opinion evidence that is related only to matters that are within the expert’s area of expertise; and

(c) to provide such additional assistance as the court may reasonably require to determine a matter in issue. O. Reg. 438/08, s. 8.

Duty Prevails

(2)  The duty in subrule (1) prevails over any obligation owed by the expert to the party by whom or on whose behalf he or she is engaged.

Expert’s Report

A prerequisite to give expert evidence at trial and also to promote settlement is that each potential witness must provide a written report. Expert reports must meet the minimum criteria set out by Rule 53.03 of the Rules. The following are the key requirements:

  1. signed by the expert;
  2. served 90 days prior to the pre-trial (or 60 days in the case of a responding report);
  3. report must contain the following (Rule 53.03(2.1)):
    1. The expert’s name, address and area of expertise.
    2. The expert’s qualifications and employment and educational experiences in his or her area of expertise.
    3. The instructions provided to the expert in relation to the proceeding.
    4. The nature of the opinion being sought and each issue in the proceeding to which the opinion relates.
    5. The expert’s opinion respecting each issue and, where there is a range of opinions given, a summary of the range and the reasons for the expert’s own opinion within that range.
    6. The expert’s reasons for his or her opinion, including,
      1. a description of the factual assumptions on which the opinion is based,
      2. a description of any research conducted by the expert that led him or her to form the opinion, and
      3. a list of every document, if any, relied on by the expert in forming the opinion.
    7. An acknowledgement of expert’s duty (Form 53) signed by the expert.

If these requirements are not met it still may be possible to provide expert evidence but only with the permission of the trial judge.  The circumstances generating the need of the expert evidence and the absence of a formal report would have to be VERY unusual for a judge to grant such permission.

The intention of the expert rule is to foster an atmosphere of presenting expert evidence where underlying facts and assumptions are made clear for the opposing parties and the court. The intention is that clarity, rather than maintaining a ‘hired gun’ approach, will enhance the expediency and efficiency of the court process. It should also promote settlement. It was the hope of the rule committee, in educating experts on their ultimate duty, that experts would be more committed to remaining objective in their opinions and empowered to be candid to the court about the limitations of their opinions.

It is important to remember that, in the context of our judicial system, experts are special. Generally speaking, a witness is not permitted to give opinion evidence but can only testify about facts within their knowledge, observations and experience. It is then up to the trier of fact to draw inferences from the proven facts. Only a properly qualified expert can provide the court with a ‘ready made inference’ which would otherwise be unavailable due to the specialized and technical nature of the subject matter.[1]

Fact or Opinion?

The line between opinion evidence and fact evidence is murky. The court has observed “[i]t is when witnesses seek to offer opinions as to the cause of the injury, it’s [sic] pathology or prognosis that the evidence enters into the area of expert opinion requiring compliance with rule 53.03”.[2]  There are occasions in which treating practitioners may be called not as experts providing opinions but as fact witnesses. These would include circumstances in which their observation of the injured plaintiff, and perhaps a description of the treatment provided, is required. This type of evidence is not considered opinion evidence and Rule 53.03 is not engaged.

A diagnosis is not always a fact. In the law of evidence, an opinion is an “inference from observed facts”. A diagnosis begins as an inference that a doctor, relying on their expertise and experience, makes from observations and other information to identify an injury or disease. As a result of additional observations or treatment, may or may not, prove to be correct. The statement of the witness does not establish as a fact that they correctly diagnosed the injury or illness – for the purposes of evidence, diagnosis remains an opinion.[3]

Experts may need to mention facts about which they have no direct proof and use them to support an opinion in testimony. These facts would then need to be proven by other evidence. It is common for experts to give testimony about facts – such as what they observed during an examination. An expert may form their opinion on second-hand information. However, the strength and weight of the expert’s opinion may erode if these facts are not otherwise established.

Giving evidence at Trial

In the rare case that a matter is not capable of resolution a trial may be necessary and you may be called upon to give your expert evidence at that trial.

Expert evidence is not admissible in all cases. The subject matter of the inquiry must be such that ordinary people are unlikely to form a correct judgment about the issue(s) unless they are assisted:[4]

What is required is that the opinion be necessary in the sense that it provide information “which is likely to be outside the experience and knowledge of a judge or jury” … the evidence must be necessary to enable the trier of fact to appreciate the matters in issue due to their technical nature. … in order for expert evidence to be admissible, “[t]he subject-matter of the inquiry must be such that ordinary people are unlikely to form a correct judgment about it, if unassisted by persons with special knowledge”.

It is the role of the judge to act in a “gatekeeper” function to ensure that, before an expert is permitted to give opinion evidence, the witness demonstrates under oath that he/she has sufficient specialized knowledge, skill or experience to give the opinion. The Mohan criteria is used by the court to determine whether expert opinion is admissible:[5]

  • The evidence must be relevant to the matters in issue.
  • The evidence must be necessary in assisting the trier of fact.
  • The absence of any exclusionary rule; and
  • The evidence must be presented by a properly qualified expert.

It is important to pause at this point to remember the importance of the expert report. An expert who does not file a ‘valid’ report, in compliance with Rule 53.03 and served within the appropriate timelines, will not be permitted to testify at trial. Secondly, an expert permitted to testify at trial will only be permitted to comment on information and documentation within the ‘Four Corners’ of the report. The actual written report of the expert witness typically does not become an exhibit in the trial. While it is common practice for counsel to provide a courtesy copy to the trial judge, the actual evidence that can be accepted by the court must come from the expert witnesses’ oral testimony.

The following provides an outline of the procedure for typical expert testimony:

  1. Court implicitly accepts that the expert opinion is admissible and in accordance with the Mohan criteria;
  2. The procedure to qualify the expert starts with a review of the expert’s CV:
    1. counsel calls the witness to the stand;
    2. counsel informs the judge as to the areas in which counsel proposes to qualify the expert;
    3. counsel examines the witness on their qualifications;
    4. counsel requests that the judge accept the witness as being qualified to provide opinions on specific subjects which counsel lists;
    5. opposing counsel is provided an opportunity to cross-examine the witness on his or her qualifications;
    6. both counsel are entitled to make submissions;
    7. the judge announces whether or not the judge accepts the qualifications of the witness on the specific subject proposed; and if it has been raised, rules on the admissibility of the opinion.
  3. Once accepted, the expert will begin testimony in examination in chief, questioned by the party that retained them;
  4. Cross-examination by opposing party;
  5. Counsel has the opportunity for a brief reply to clarify testimony, if required.

Remember that your CV will, in almost every case, become an exhibit at trial. Counsel, and the court, will spend significant time reviewing your CV during the qualification stage set out at Part II above. Take the time to ensure that your CV is organized, accurate and aesthetically pleasing. Keep your CV updated, edit for clerical and other errors in the document. Nothing is worse than starting off your testimony after being taken to task for an inadequate or inaccurate CV.

Use of Documents during Testimony

Experts are permitted to use documents while testifying. Experts commonly refer to their own report on the subject, in addition to any literature and notes recording their own tests, investigations and research. However, expert testimony is limited to the substance of the report filed.

During cross-examination, counsel is allowed to ask questions relating to other tests, materials and issues not in the report. However, before counsel can cross-examine on scientific or technical literature, the expert must agree that the specific piece of literature is authoritative. Frequently counsel will also request access to the expert’s complete file prior to the expert testifying, including preliminary or draft reports, notes and any other documents or communications used to prepare the final report.

Do’s and Don’ts

Speak directly to the Trier of Fact

Engage directly with the judge and jury. Speak to them, make sure they are following along and understanding what you explaining. You want the trier of fact to be engaged and to feel like they have a relationship with you directly.

Educate the Judge and Jury

While providing your testimony, think about what you need to teach the judge and jury. A good expert can explain their subject matter to a group of 12 year olds. Use demonstrative aids to help them understand.  Most jurors need a multi-dimensional educational process. They have to hear AND see the material to retain your point.

Presentation is Important

Your opinion will only be useful if it is accepted. Ensure that you present as a confident, professional, well organized witness. Dress like you deserve to be respected. Speak to be understood. Engage with the jury and make sure that they are following along. Aim to present your evidence in a clear, cogent and organized fashion.

Stay within your field of Expertise

Do not get caught providing opinion evidence that is outside your area of expertise. An opinion that goes beyond the scope of your expertise will open you up to cross-examination. This can significantly undermine otherwise impeachable testimony. Keep your guard up, focus your answers to provide accurate and cogent evidence.

Prepare yourself

Know your file. Review your complete file before you meet with counsel. Bring your file, including notes, photographs etc. to your preparation meeting. Review your report, and the opposing expert’s report(s), before you attend in court. If more recent medical documentation has been provided, review that to understand any changes in the plaintiff’s condition that may amend your opinion. 

Prepare for Cross-examination

The best preparation for cross-examination is to know your, and the opposing expert’s, reports inside and out. Understand the differences in the assumptions and approaches between your report and theirs. Be prepared to explain these differences in a clear manner that is understandable to the court and jury. Review the authoritative texts and studies cited in your reports. Be careful about admitting that a text is authoritative too readily – if the text is not authoritative, you cannot be cross-examined about it. If you are not sure, hold firm.

Ensure that part of your preparation involves reviewing updated reports that may change your opinion. In reviewing these reports, think about how these new facts impact the opinions in your report. Be prepared for cross-examination on these issues.


Remember that you are there to assist the court in understanding a more difficult element of the case. You are a fair, objective and non-partisan professional. Demonstrate to the court that you are there to help – candour, humility, openness and cooperation are key. Do not get angry or argumentative with opposing counsel. If your personal integrity is challenged, defend yourself but in a controlled and dignified way.

Know that opposing counsel will often ask you about the opposing party’s expert. You will be asked whether you respect the opposing party’s expert. Formulate a plan to answer this question.

As a final thought, be confident in your expertise.  A lawyer, no matter how skilled, is not going to know more about your area of expertise than you. 

Good Luck.

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