Common Mistakes in Expert Witness Reports

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What Makes a Strong Report?

An expert witness report can be an invaluable tool in strengthening a case. Even before a matter is heard in court, a strong report can be helpful in negotiations, potentially assisting the parties involved in avoiding a lengthy and expensive trial.

But what makes a good expert report and what are the common mistakes that can make a report less effective and, at worst, inadmissible in court? This article examines some typical issues that arise in the expert witness report process, and how to avoid them.

The Letter of Instruction

The first stage in the preparation of an expert witness report is generally a letter of instruction prepared by the instructing solicitor, which accompanies a brief of documents for the expert to review. Among other things, the letter of instruction will contain the questions the expert is required to address.

When briefing an expert, lawyers should keep in mind that an expert’s role in litigation is to give impartial opinion evidence about issues in the matter. Solicitors should provide a copy of the relevant Code of Conduct, outlining all the expert’s obligations.

Problems can arise when a letter of instruction is too broad in its scope and doesn’t focus on the specific issues that the instructing solicitor needs the expert to address. This can result in a report that does not effectively assist the lawyer’s case.

For example, a question such as, “What are common causes of building collapse?” could invite pages of information not relevant to the matter.

A better question would be, “In the case of the subject building collapse, what do you consider the most likely cause and why do you consider this cause most likely?”

Another problem can occur when lawyers draft questions that require an expert to provide comment on an area outside their area of expertise. For example, a slip and fall expert may not have the requisite expertise to comment on how much it would have cost for a company to have installed handrails or less slippery flooring. Conversely, asking a builder this same question may be more reasonable, as builders often work from industry standard pricing guides.

Finally, asking an expert the question, “Are there any other matters of relevance you wish to comment upon?” can have unintended consequences. While it can allow an expert to add useful information not covered by other questions in the letter of instruction, there are situations where an independent expert will make comments that, while reasonable and objective, adversely affect a lawyer’s case.To some extent, this question is also captured in the standard declaration from experts that they have made  all enquiries considered appropriate, and their report contains reference to all matters they consider significant. Ideally, all the information the instructing solicitor requires the expert to comment on should be addressed by specific questions.

The letter of instruction should also contain the background of the case and all relevant facts and assumptions the expert is required to accept. It is important that the brief of documents and/or materials to assist the expert aligns with the assumptions the expert is asked to make. Problems can arise where the expert finds information that contradicts the assumptions provided in the letter of instruction. For example, if an expert is asked to assume that a plaintiff hurt their back when lifting a crate on their own but then the expert interviews the plaintiff and is told that they lifted it with a second person, it could materially alter the expert’s opinion about the cause of the injury.

Lawyers should also specify a timeframe for delivery of the report. It is a stressful and unwelcome situation where the due date for service is imminent, but the expert is unaware and still far from completion of their report.


The standard case reference for expert reports is the judgement of Heydon JA (as he then was) in Makita (Australia) Pty Ltd v Sprowles (2001) 52 NSWLR 705. His Honour made clear that an expert must identify and prove the factual basis of their opinion, and must explain how their field of specialised knowledge, on which their opinions are based, applies to the facts and assumptions in their report.

One of the more common mistakes made in expert report writing is when experts present opinions that are not supported by reasoning and reference to evidence or assumptions that have been provided. Without clear reasoning, experts risk parts of the report, or even the entire report, being objected to on the grounds of inadmissibility, or else given little weight by the court. While some questions may only require a ‘yes’ or ‘no’ answer, they should be followed by a question that asks the expert to fully explain their reasoning for that answer.

This concept has been adapted into most of the various Expert Witness Codes of Conduct used in each jurisdiction.  For example, the Federal Court Practice Note on Expert Evidence states:

“An expert witness’ opinion evidence may have little or no value unless the assumptions adopted by the expert (i.e. the facts or grounds relied upon) and his or her reasoning are expressly stated in any written report or oral evidence given.”[1]

It should be ensured that experts clearly link their opinions to their study, training, and experience. An expert is unlikely to be criticised for consistently and clearly outlining how their experience has formed the basis of their opinion (e.g. “Based on my 20 years of experience in the industry, it is my opinion that…”)

Area of Expertise

It is a common occurrence for experts to provide comment on areas beyond their expertise.  This is generally unhelpful, and these types of comments may be challenged in court on the basis of irrelevancy and/or being outside the scope of the report. An example could be an expert cosmetic surgeon being asked to comment on a botched operation, and also offering opinions on the psychological impact of the incident. As noted above, conferencing with the expert before they commence work on a report can also clarify for an expert as to what areas they are required and able to provide expert comment.

Another area of concern is when an expert offers legal conclusions, even if not intentionally. This might include comments like, “The defendant was clearly negligent” or “This was a breach of their duty of care”. Comments like this amount to legal conclusions and are in most cases not allowable for experts who have no legal training. Lawyers should not frame questions in a letter of instruction that go to the ‘ultimate issue’ in dispute and should not draw experts into making such conclusions (e.g. ‘In your opinion did the defendant breach its duty of care?’).


Avoiding common mistakes throughout the process of engaging and drafting an expert report can produce a well written report that clearly answers specific questions that will not only assist a lawyer’s case but also fulfil the primary aim of expert evidence: to assist the court in understanding complex or technical issues and assist in the resolving of issues in dispute.

To read more about the problem with ‘Ultimate Issue’ questions, please click here.



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