Assumptions v Findings: Which Should Prevail?

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When experts are briefed with assumptions that contradict their own findings.

It is common for lawyers to brief an expert in a letter of instruction with a set of assumptions about the matter. The expert will then be instructed to use those assumptions as the basis of their opinion. It will then be up to the lawyers to prove those assumptions in court so that they become, in the court’s eyes, ‘facts’.

But what happens in a situation where the expert is asked to use assumptions that are contrary to what they know or believe to be correct? Should the expert rely upon the assumptions, or rely upon their own findings?

Example: Inconsistencies in Client Interview

In some cases, an expert may interview and/or examine a client before preparing a report. In this example, during the interview, the client outlines a set of circumstances in which their injury occurred. The client states that their injury was caused as follows: “I was lifting a crate with a co-worker and I felt my back go out”.

However, in the letter of instruction, the expert finds the following:

You are to assume that our client was instructed by his supervisor to lift a crate on his own, at which point, he felt his back go out.”

If the expert prepares their report based on what they were told during the client interview, the report may turn out unfavourable for the lawyers, or at the very least substantially different to the report that the expert would write based only on the provided assumptions. If the expert proceeds based on the assumptions provided to them, ignoring their own findings, and the matter gets to court, the expert’s truthful response, if tested on the assumptions in the witness box, will be that the client told them he lifted the crate alone. This immediately gives less credence to their report.

The argument for following the provided assumptions is that it is not up to the expert to prove the truth of the assumptions; that this is the lawyers’ job in court. Indeed, the client may have told a different story to the lawyers or remembered things differently over time.

Another example is where an expert receives a brief of documents and/or materials to assist them in preparing their report but within that material the expert finds information that contradicts the assumptions provided in the letter of instruction. Again, by using the findings in the brief rather than the assumptions, the report may be substantially different.

In any case, when an inconsistency between the assumptions and findings does occur, the expert’s first response should be to communicate with the instructing lawyer and discuss the issue. It is important for the lawyer and the expert to be on the same page. It may be that the lawyer needs to reissue the letter of instruction with updated assumptions.

However, the expert’s overriding duty is to the court, not to the lawyers or their client. That means giving their honest and independent opinion, based on the knowledge they have and the facts as they understand them.

Using the example above, if the expert writes the report based on the information from the client rather than the assumptions provided by the lawyer, and the lawyer is then unhappy, is there fault on either side?

The Case Law

There are numerous cases, including Makita (Australia) Pty Ltd v Sprowles [1] and Dasreef Pty Ltd v Hawchar [2] that clearly outline the duty of an expert and the issues involved with unproven assumptions in expert witness reports.

Heydon J, in Dasreef, addressed several of these issues in his judgement. For example, at [66] he said:

“An expert opinion is not admissible unless evidence has been, or will be, admitted, whether from the expert or from some other source, which is capable of supporting findings of fact which are sufficiently similar to the factual assumptions on which the opinion was stated to be based to render the opinion of value.”

Later in the decision at [90] his Honour noted that:

“If the expert’s conclusion does not have some rational relationship with the facts proved, it is irrelevant. That is because in not tending to establish the conclusion asserted, it lacks probative capacity. Opinion evidence is a bridge between data in the form of primary evidence and a conclusion which cannot be reached without the application of expertise. The bridge cannot stand if the primary evidence end of it does not exist. The expert opinion is then only a misleading jumble, uselessly cluttering up the evidentiary scene.”

Unless there is evidence to support the assumptions in an expert report, the report will be given little weight by the court, or even be deemed inadmissible. The onus is firstly on lawyers to give proper instructions in their letters of instruction, with assumptions that can be proven in court. It should also be remembered that an expert witness is bound by the Expert Code of Conduct to be independent and, if they have knowledge that will assist the court, even if it contradicts the assumptions they have been given, then they are required to use it in preference to assumptions provided. As Heydon J says in Dasreef at [102]

 “Is an opinion tendered under s 79 inadmissible unless there was evidence, admitted or to be admitted…capable of proving matters sufficiently similar to the assumptions to render the opinion of value? The correct answer is in the affirmative.”

In Beagle v Australian Capital Territory and Southern New South Wales Rugby Union Limited[3],  an expert report was deemed to be unreliable when the assumptions in it were found to be significantly different to those briefed to the expert.

The plaintiff, John Beagle, sought remuneration for assistance that he allegedly gave to the Australian Capital Territory and Southern New South Wales Rugby Union. Mr Beagle alleged that he played a significant role in securing a sponsorship deal for the union and was therefore entitled to a commission of 10% to 35% of the value of the sponsorship.

He relied upon the report of an expert in the area of corporate sponsorship to indicate the proper amount of compensation he was entitled to. Associate Justice Mossop in the ACT Supreme Court found that the expert’s assumptions were contrary to the evidence put forward during the trial, and the source of her assumptions was unclear:

“There was a significant degree of uncertainty as to precisely where the ‘facts and assumptions’ identified in her first report had come from. Several critical matters were clearly not derived from the terms of her letter of instructions.” [174]

“The inadequate foundation for her opinion extended to her identification of various factors in favour of higher levels of remuneration … None of these considerations are consistent with the facts.” [175]

Other issues identified by Mossop AsJ in the report included:

  1. The report suggested that Mr Beagle provided “intelligence[that] would have been critical” to the sponsorship. [50] However, this was contrary to the court’s findings that the information he provided was available in a basic internet search and consequently could not be considered crucial to closing the deal;
  2. The expert appeared to have access to information similar to that within the affidavit of Mr Beagle but before that affidavit was prepared;
  3. The expert was asked to adopt assumptions in the letter of instruction which were not proven at trial. This meant the Court gave less weight to her report.

Given that the expert had used assumptions not provided to her and that the assumptions were not proven by the evidence, the Court did not accept the expert’s evidence as reliable for establishing what Mr Beagle’s reasonable remuneration should have been.


Expert evidence usually relies upon facts and assumptions briefed to the expert. When those are contradictory or cannot be proven in court, the expert evidence may be given less weight or even be inadmissible.

Lawyers and experts should work together to establish a clear understanding of what facts and assumptions are going to be relied upon and are provable before the expert commences writing their report. It is inadvisable for experts to blindly follow the assumptions that they have been given, when it appears to them that the facts are different to how they are stated, as this can do more harm than good in a case.

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[1] [2001] NSWCA 305.

[2] [2011] HCA 21.

[3] [2016] ACTSC 271

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