When Expert Witness Testimony and Expert Reports Collide

Some experts are of the view that the opinions expressed by them on the witness stand at trial do not necessarily need to align with the opinions expressed in their expert report. This disconnect between report content and trial testimony is fundamentally wrong and is contrary to law, the duties of an expert, and trial fairness. This article explores opinion incongruency with recent case examples.

Experts are asked to evaluate relevant data and research, undertake such examinations and testing as are necessary, and formulate opinions within their area of expertise for the benefit of the parties, their counsel, and the court. These opinions are often pivotal for counsel in determining whether to resolve a case or take it to trial. Given the importance of expert reports in case assessment and trial strategy, counsel must be satisfied that the reports under consideration are full, fair, frank, and state any qualifications to the opinions expressed therein. It should not be the case that counsel can expect to extract different opinions from experts on the witness stand.

These report and testimony incongruency issues tend to arise when experts have issued their report based on the information available to them and thereafter receive new information which may cause them to vary their opinion to some degree. What should happen in these circumstances is that experts will consider the new information and if it makes a difference in their evaluation and opinion, they will issue a supplemental report that delineates the new information received and considered. and any changes in their opinion stemming from the new information. The supplemental report is then disclosed in the prescribed manner.

In two recent UK cases, this is not what occurred and the courts expressed concern for this poor practice. In Mew v. General Dental Council [2026] EWHC 1116, an expert wrote and submitted a report that did not set out the limitations of the data analyzed and the expert impliedly justified this approach on the basis that he would explain the limitations in his testimony when questioned about them. Aside from the obvious failure to disclose problem, this approach means that if counsel do not ask the right questions, the limitations known to the expert will remain unstated. Similarly, in Greening-Steer v. Ainge [2026] EWHC 1239, an expert wrote a report and was subsequently provided with new information that significantly changed his opinion. However, rather than amend his written opinion via a supplemental report, he preferred to wait until cross examination to state his modified opinions. Only then was his fundamentally altered opinion fully disclosed. It is unclear whether these failures by the experts to disclose their full opinions in a timely fashion were caused by inattention, lack of diligence, or a misguided sense of gamesmanship. In both cases, the courts noted that this practice was contrary to law and therefore improper.

In UK civil cases, the Civil Procedure Rules state that an expert’s report must represent the expert’s ‘true and complete professional opinions on the matters to which they refer’ (CPR PD35.3.3) and that ‘if the expert is not able to give an opinion without qualification, state the qualification’ (CPR PD35.3.2(8)). Similar obligations are set out for criminal cases in Rule 19.4 of the Criminal Procedure Rules. Even in jurisdictions that do not have express rules to this effect, experts have a professional and ethical responsibility to produce complete reports that clearly set out their opinions and any qualifications, and to disclose modified opinions in a timely fashion.

The reasons for these rules are clear. Counsel cannot effectively evaluate the substance and significance of an expert report if they cannot be certain that the report represents the fully considered and final opinion of the expert. In the cases cited above, if counsel had made resolution decisions based on the reports as written, they would have been doing so based on outdated, incomplete, and sometime erroneous opinions. That would do a disservice to at least one party in the litigation. Since most cases are resolved short of trial, having complete and current opinions set out in expert reports is essential. It must never be the case that if counsel poke an expert in just the right place on the witness stand or ask the correct questions, the expert will offer new or varied opinions. The courtroom should not be the place for surprises.

The takeaway from these cases is to emphasize that experts must write complete and current reports that fully set out their opinions and any qualifications thereto. Should experts be given or learn of new information that has the effect of qualifying or changing the previously provided opinion, a supplemental report must be written and disclosed to the appropriate parties. Only then will an expert have complied with their legal and ethical obligations. This will in turn allow counsel to make informed decisions about the strength of their case and that of the opposing party and ultimately whether there is value in proceeding to trial, rather than seeking an earlier resolution.