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

A Well Documented and Supported Opinion from a Subject Matter Expert



Last Updated: August 23 2026

Question: What should an Ontario paralegal or solicitor check before filing an expert witness report with a court to maximize admissibility?

Answer: In Ontario, a paralegal at DK Legal Practice can help you confirm your expert witness report is likely admissible by aligning it with the Mohan factors (relevance, necessity, no exclusionary rule, and proper qualification) and the White Burgess Langille Inman v. Abbott and Haliburton Co. balance of risks and benefits before filing,  then tightening scope with the requesting lawyer or paralegal, clearly referencing supporting evidence in appendices or schedules, and presenting unbiased, organized opinions using readable, audience-appropriate language,  for a document checklist and drafting help in civil litigation, call (416) 906-6663 to get started today.

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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 A proper, well crafted, expert witness report can make the difference between success or failure within a lawsuit; and accordingly, avoiding mistakes within an expert report is of utmost importance so to ensure that the expert report is admissible, and may be relied upon, within the court proceeding. If an expert report is deemed inadmissible, such may jeopardize the legal case.

Note: Please contact DK Legal Practice by phone at: (416) 906-6663 to discuss any specific questions that you may have.

The Law
Admissibility

For a court to admit an expert report as evidence the expert report must meet the criteria of admissibility.  The criteria was summarized within the case of 2599475 Ontario Inc. v 2549445 Ontario Inc., 2023 ONSC 3508, where it was said:


[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

Prior to drafting a report, the expert should review the required scope of the report with the representative, being the lawyer or paralegal, who seeks the report.  Depending on the nature of the court case, the report may require broad details or may require a narrow focus.  In some circumstances, a brief report is adequate; and in other circumstances a more lengthy report is necessary.  Forgoing this review can result in the waste of preparing an insufficient report or the waste of preparing an excessive report.  Either is a waste of time and therefore money.

A proper expert report includes, and properly references, the evidence documents that support the expert opinions and conclusions. 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. A bibliography referencing any authoritative documents or studies that are used to support opinions and conclusions should also be included.

The report should read clearly with ease and appear professional and proofed for proper spelling, grammar, and punctuation.  The expert should write in a style directed towards the intended audience, balancing need that the report be understood by an average layperson without belittling highly intelligent judges and lawyers or other experts.  Reports should omit or limit industry specific jargon and abbreviations; but, if such is necessary, include definitions 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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