Expert Witness Report: A Well Documented and Supported Opinion from a Subject Matter Expert | Byfield Legal Services


Last Updated: August 22 2026

Question:How do I prepare an expert witness report document that is likely admissible in Ontario court litigation?

Answer:Byfield Legal Services can help paralegals and litigants in Ontario prepare an expert witness report that meets the admissibility requirements (relevance, necessity, no exclusionary rule, and a properly qualified expert) and then balances risks and benefits once the four Mohan factors are reviewed by the court.   The report should clearly state the expert’s independent, objective opinions grounded in reviewed evidence, include properly referenced appendices or schedules, and add a short bibliography for authoritative sources used.   It should also match the required scope from the requesting lawyer or paralegal, use a clear, organized lay-friendly tone with defined technical terms, and follow practical court-ready formatting like a cover page, index, headings, consistent page numbering, and a summary of conclusions at the front.   For help getting your Ontario expert report drafted or revised to reduce inadmissibility risk, call (289) 927-7365.

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Understanding the Requirements When Preparing an Expert Witness Report Document for Court Litigation Purposes

Expert Witness Report: A Well Documented and Supported Opinion from a Subject Matter Expert Various best practice habits when preparing expert reports can help ensure a well-crafted document that is both effective and efficient for use within the courtroom. Indeed, a quality expert report can be the difference in making or breaking a case; and accordingly, the choice of expert, the style of report, and the manner used when expressing expert opinion, require careful review.

Note: Please contact Byfield Legal Services by phone at: (289) 927-7365 to discuss any specific questions that you may have.

The Law
Admissibility

When reviewing whether to admit an expert report as evidence, a court will seek to ensure that the report meets the required admissibility criteria.  Expert evidence admissibility criteria summarized within 2599475 Ontario Inc. v 2549445 Ontario Inc., 2023 ONSC 3508, whereas it was stated:


[35]  The test for the admissibility of expert evidence is based on the criteria set out in R. v. Mohan, 1994 CanLII 80 (SCC), 1994 SCC 80, as follows:

a)  Relevance;

b)  Necessity in assisting the trier of fact;

c)  The absence of any exclusionary rule, apart from the opinion rule itself; and

d)  A properly qualified expert.

[36]  In White Burgess Langille Inman v. Abbott and Haliburton Co., 2015 SCC 23, [2015] 2 S.C.R. 182, the Supreme Court divided the admissibility requirement into two distinct steps.  First, a trial judge considers the four Mohan criteria.  Then the judge balances the potential risks and benefits of admitting the expert evidence.

Quality Expert Reports
Communicate Clearly and Effectively

Before the work of preparing a report begins, the expert should obtain a clear and concise understanding of the scope of detail required within the report by communicating directly with the lawyer or paralegal who is seeking the report from the expert; whereas, depending on the nature of the litigation, the required statements within the report may vary between just a general summary expert viewpoint or a highly detailed expert opinion. Failing to conduct this review may result in the expert wasting the effort of preparing an excessively detailed report or useless preparing an insufficient report; and whereas both possibilities may result in a waste of time, such is also a waste of money.

It is also necessary that the supporting evidence documents reviewed by the expert are adequately included and properly referenced.  The expert opinion contained within a report should, and perhaps must, refer to supporting evidence documents such as pictures or testing results, among other things, attached as appendices or schedules.  Additionally, a bibliography to summarize any authoritative documents used for research should be included.

An expert report should express opinions clearly and concisely while being easy to read in an organized and professional layout with spelling, grammar, and punctuation, carefully reviewed for correctness.  The expert report should be written with a vernacular and tone that is consistent with the language of the intended audience whereas, for example, differences may be required for legal cases heard by a judge versus legal cases heard by a jury; and accordingly, language should remain within the understanding of the average layperson while avoiding belittlement of judges, lawyers, and other experts.  Reports should be careful to limit the use of industry specific jargon or abbreviations known only to those with expertise within the relevant field; and, where industry jargon or abbreviations are necessary, a reference table or key to provide definitions should be provided within the report.

More Hints

  • Use professional letterhead;
  • Use a cover page as well as an index or table of contents;
  • Use a curriculum vitae or resume to summarize the qualifications and credentials that confirm that the expert is genuinely an expert;
  • Use an introduction page to outline when, and from whom, the report was requested;
  • Use 12-point font and double spacing;
  • Use headings;
  • Use short, concise paragraphs;
  • Use a unique reference number for each page, table, chart, and appendix;
  • Use a definitions page or section to define technical language and industry specific jargon or abbreviations; and
  • Use an upfront summary page to outline final conclusions or opinions.

Things to Avoid

A biased or slanted expert report can be disastrous – and should be!  The expert is expected to provide opinion as a friend of the court for the primary purpose of assisting the court in the truth finding effort.  The expert, while having an opinion that may favour one litigant over another, must base the opinion on authentic reasons rather than as friend of a litigant.

  • Avoid absolute wording and phrasing;
  • Avoid words that appear as hedging or guessing;
  • Avoid critical commentary regarding credibility of other experts;
  • Avoid a scientific and clinical tone by keeping the tone friendly and personable;
  • Avoid going into details beyond the purpose of the report;
  • Avoid unnecessary adjectives or superlatives.

Summary Comment

The expert preparing an opinion report must remain attentive to the balance required when obtaining input from a legal representative whereas the lawyer or paralegal seeking the report will best understand the factual concerns and legal issues that relate to why the report is needed and can explain the desired scope; however, the findings and opinions as stated within the report must always remain the genuine unslanted view of the expert as author of the report.

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