The Rebuttal Expert Report: 4 Steps For Critiquing An Opposing Expert

Anjelica Cappellino, J.D.

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— Updated on April 11, 2022

The Rebuttal Expert Report: 4 Steps For Critiquing An Opposing Expert

If an expert is necessary to prove a case theory, chances are the opposing party will also retain their own expert. But no matter how sound one side’s legal theories and expert opinions appear to be, an opposing expert’s conclusions will likely present a number of challenges to those arguments. However, an unfavorable opposing opinion does not mean that your own expert’s opinion is incorrect or incomplete. Oftentimes, a rebuttal expert is used to rehabilitate your side’s arguments while discrediting the opposing expert’s opinions. Like the initial expert opinion, a rebuttal expert may draft a report of their findings. However, the substance, strategy, and timing of this report serve different functions. When offering a rebuttal expert report, it is important to keep these four considerations in mind.

1. Consider the Scope of the Rebuttal

Rebuttal evidence, as the term suggests, rebuts the evidence offered by an opposing party. It is limited “to that which is precisely directed to rebutting new matter or new theories presented by the defendant’s case-in-chief.” Rebuttal evidence is properly admissible when it will “explain, repel, counteract or disprove the evidence of the adverse party.” It is not “an opportunity for the correction of any oversights in the plaintiff’s case in chief.”

In the context of rebuttal experts, Rule 26(a)(2)(D) of the Federal Rules of Civil Procedure allows the evidence to be admitted if it “is intended solely to contradict or rebut evidence on the same subject matter,” and must be disclosed 30 days after the other party’s disclosure. A “rebuttal expert report is not the proper place for presenting new arguments unless presenting those arguments is substantially justified and causes no prejudice.” Courts have wide discretion in the admissibility of expert evidence and can strike a rebuttal expert’s disclosure if it goes beyond its permissible scope. For example if the report “fails to provide scientific disagreement” with the opposing expert’s construction, or it contains “substantial new data and information” exceeding the original report.

Although an expert’s rebuttal report should not contain new opinions or information which contradicts the initial report, it need not be stricken if it is “an elaboration of and [is] consistent with an opinion/issue previously addressed” in the expert’s initial report. A report “may cite new evidence and data so long as the new evidence and data is offered to directly contradict or rebut the opposing party’s expert.” In other words, there is no “bright line rule” that every expert opinion “must be included in a preliminary report, or forever be precluded.” That being said, it is important to be mindful of the content of the rebuttal report so that it does not go beyond its permissible scope.

2. Learn the Opposing Expert’s Opinion

Before attacking the opposing expert, the first step is understanding the opinion and the basis on which it is formed. A critical analysis is necessary to ensure that the opinion is challenged in a meaningful way. It is also important in understanding which aspect of the opposing expert’s argument is incorrect or contradictory to your side’s position. All key ideas, methods, and assumptions used in the opposing expert’s opinion should be analyzed to determine where the two parties diverge. It is not always as straightforward as simply stating that all aspects of your adversary’s analysis are incorrect.

More often than not, there is a more nuanced difference between the two. In cases where testing and experiments were conducted, it can be helpful to recreate those same conditions and variables to pinpoint why the opposing side reached a different conclusion. Overall, like any strong rebuttal, the opposing arguments need to be fully understood before they can be successfully challenged.

3. Challenge the Methodology

As aptly stated in the seminal United States Supreme Court decision, Daubert v. Merrell Dow Pharmaceuticals, Inc., there are several factors that should be considered by a court when determining the admissibility of expert testimony. While no one factor is deemed more important than another, it would be remiss to ignore the significance of the first listed factor: whether the expert’s technique or theory can be or has been tested. That is, whether the expert’s theory can be challenged in some objective sense, or whether it is instead simply a subjective, conclusory approach that cannot reasonably be assessed for reliability.

The takeaway point of this factor is that admissibility rests on methodology as opposed to conclusions. In other words, an expert’s opinion should derive from a reliable, objective methodology. In accordance with this factor, the methodology of an opposing expert’s opinion should be critiqued first. The ways in which a specific methodology can be challenged are as numerous as the fields of expertise themselves. Does your rebuttal agree with the type of testing conducted? If the type of testing was correct, is the potential rate of error accurate? Do peer review and the opposing expert’s reputation in the scientific community factor in? Were inconsistent testing or analysis methodologies utilized? By focusing on the weaknesses or inconsistencies in the opposing expert’s methodologies, a rebuttal report becomes substantially more persuasive.

4. Provide Alternatives

While rebuttal reports are typically more limited in scope, sometimes they might need to state new alternatives in order to effectively rebut the opposing expert. As held in Deseret Management Corp. v. U.S., a “rebuttal expert may introduce new methods of analysis in a rebuttal report as long as the new method is offered to contradict or rebut an opposing party’s expert.” In Deseret, for example, a rebuttal report applied a different valuation method in estimating the value of a license at issue. The court found that the report was properly submitted, even though it applied, a “new, alternative theory,” because both reports addressed the same subject matter and the rebuttal report was offered to contradict the opposing party’s report. Therefore, rebuttal reports need not avoid alternatives or new means of testing if they are offered to contradict the opposing side’s arguments.


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