Rebuttal Reports and Supplementation

Two clocks, one duty, and one sanction

A rebuttal report is an expert disclosure “intended solely to contradict or rebut evidence on the same subject matter identified by another party under Rule 26(a)(2)(B) or (C).” Supplementation is the continuing duty under Rule 26(e) to correct or complete a disclosure already made. Both live in Rule 26 of the Federal Rules of Civil Procedure and are enforced by Rule 37(c)(1). They matter more in a search case, because the data behind the opinion keeps changing after the report is served.

The clocks come from Rule 26(a)(2)(D). A party makes expert disclosures “at the times and in the sequence that the court orders.” Absent a stipulation or order, affirmative disclosures are due “at least 90 days before the date set for trial or for the case to be ready for trial,” and rebuttal evidence is due “within 30 days after the other party's disclosure.” Both are defaults that a scheduling order displaces.

The duty comes from Rule 26(e). Rule 26(e)(1)(A) requires a party to supplement or correct a disclosure “in a timely manner if the party learns that in some material respect the disclosure or response is incomplete or incorrect.” Rule 26(e)(2) adds, for a reporting expert, that the duty “extends both to information included in the report and to information given during the expert's deposition,” and that “Any additions or changes to this information must be disclosed by the time the party's pretrial disclosures under Rule 26(a)(3) are due.” Rule text: Cornell LII.

What a rebuttal report may do, and what it may not

The Eighth Circuit stated the function in Marmo v. Tyson Fresh Meats, Inc., 457 F.3d 749 (8th Cir. 2006): “The function of rebuttal testimony is to explain, repel, counteract or disprove evidence of the adverse party.” Rebuttal “is not to be used as a continuation of the case-in-chief.”

The district courts have filled in the test. In Spring Creek Exploration & Production Co. v. Hess Bakken Investment II, LLC (D. Colo. April 21, 2016), the magistrate judge collected the standards: rebuttal experts “cannot put forth their own theories; they must restrict their testimony to attacking” the adversary's; opinions that “do not directly contradict or rebut the actual contents of that prior report” are not rebuttal; and “A rebuttal report is not the time to change methodologies to account for noted deficiencies.” The test is “whether a responding expert witness limits himself to ‘poking holes’” in the other side's report.

On “same subject matter,” the same order quoted the view that courts “should not place an overly restrictive interpretation on this operative phrase.” The subject is the opposing expert's actual report, read generously, but the rebuttal must engage what that report says.

Poking holes versus telling a different story about the traffic

The plaintiff's expert typically serves a before-and-after analysis (a comparison of organic traffic, meaning visits that arrive from unpaid search results, before the defendant's conduct against the period after it) and attributes the difference to the conduct. A defense expert who shows that the “before” window was unrepresentative, that the data source changed mid-period, or that a known confound was not controlled for is poking holes.

A defense expert who offers an alternative explanation is doing something else. A Google core update (a broad change to Google's ranking systems, announced by Google and rolled out over days or weeks) that landed in the “after” period, or a site migration (a change of domain, platform, or URL structure) the plaintiff ran at the same time, supplies a competing cause, and a competing cause is an affirmative theory. The Spring Creek court observed that defense experts responding to a damages claim “are entitled to present their own new theories concerning Plaintiffs' alleged damage calculations,” while cautioning that “those opinions may be considered affirmative in nature.”

In my experience the defense rarely gets to choose cleanly; the hole and the alternative story are often the same fact. Pointing out that the plaintiff's expert ignored a core update is a criticism; stating that the update caused the decline is a theory. Disclosing the alternative-causation expert on the affirmative schedule, or obtaining a stipulation, avoids the argument.

Supplementation is a duty, not a right

The cases describe Rule 26(e) with a consistent set of phrases, collected in Obesity Research Institute, LLC v. Fiber Research International, LLC (S.D. Cal. August 4, 2017). It “creates a ‘duty to supplement,’ not a right.” Supplementation “means correcting inaccuracies, or filling the interstices of an incomplete report.” It “does not give license to sandbag one's opponent with claims and issues” that belonged in the original report.

The test that matters for changing data is the one applied in Isaac v. Forcillo (D. Nev. April 14, 2022): “whether the supplemental information corresponds to a prior Rule 26(a) disclosure,” and whether the information was available when the initial disclosure was due. A supplement based on an MRI produced two days before the deadline passed both. Supplements based on records produced months earlier failed the second, because the expert “had or should have had access” to them. The court found no bad faith, reopened expert discovery for limited depositions, and shifted the costs to the offending party's counsel.

The Northern District of Illinois drew the other line in McDavid, Inc. v. Nike USA, Inc. (N.D. Ill. April 23, 2013): whether a “supplement” introduced a new theory the other side had no reason to anticipate, and whether new facts or a change in law justified it. The court struck one supplemental report in its entirety, struck the portions of a damages report that set out an alternative apportionment theory, and allowed new material where it was justified or the prejudice limited.

The deposition, the last day, and the cost of waiting

Rule 26(e)(2) reaches what the expert said under oath. The 1993 Note says the duty covers “any material changes made in the opinions of an expert from whom a report is required, whether the changes are in the written report or in testimony given at a deposition.”

The last day is set by Rule 26(a)(3). Pretrial disclosures are due “at least 30 days before trial” unless the court orders otherwise, and Rule 26(e)(2) requires additions or changes to expert information by that time. The Note says supplementations “should be made at appropriate intervals during the discovery period, and with special promptness as the trial date approaches.” In Opengate Capital Group LLC v. Thermo Fisher Scientific Inc. (D. Del. February 1, 2016), the Special Master denied leave to serve a supplemental damages report where the party had waited about two months after learning of the gap, treating the delay as willful though not in bad faith, and noted that parties “may not use their obligation to supplement as an excuse to violate the clear terms of a Scheduling Order.”

Search data that moves while the case is pending

No reported decision I have found addresses supplementation of Search Console, GA4, or rank-tracking data. What follows is analysis.

Search Console's rolling window. Google Search Console (Google's free reporting service showing a site owner how the site performs in Google Search) makes performance data available for a rolling sixteen-month window. A supplement that refreshes an existing analysis with the months that have arrived since the report “corresponds to a prior Rule 26(a) disclosure.” The risk runs the other way: months drop off the front of the window, and an expert who did not export the data at the time of the report, and later cannot reproduce the figures, has a reliability problem that supplementation does not cure.

A core update lands mid-case. Post-update traffic data is information the party “learns” after the report, within the words of Rule 26(e)(1)(A). A supplement that reports it and states whether the existing causation opinion is affected is a correction. A “supplement” that uses the update as the occasion to adopt a different damages model or causation theory is the methodology change that Spring Creek and McDavid describe, and it deprives the opponent of the 30-day rebuttal window.

GA4 retention purges. Google Analytics 4 (Google's current web analytics platform, which records visits and on-site events) lets the property owner set event-data retention to “2 months” or “14 months.” Per Google's documentation, “The data retention setting only affects explorations and funnel reports,” not standard aggregated reports. If the opinion rests on an exploration whose events will age out before trial, the underlying data has to be exported when the report is written. A later substitution of a standard report for a purged exploration changes the data basis of the opinion and must be disclosed under Rule 26(e)(2).

Rankings that move. Rank-tracking data (position records collected by a third-party tool that queries the search engine on a schedule) gathered after the report is new information, and reporting it is supplementation.

Rule 37(c)(1) is automatic, and harmlessness is the whole fight

A party that fails to disclose or supplement “is not allowed to use that information or witness to supply evidence on a motion, at a hearing, or at a trial, unless the failure was substantially justified or is harmless.” The 1993 Note calls this “a self-executing sanction.” Bad faith is not required; the Seventh Circuit formulation quoted in McDavid is that exclusion “is automatic and mandatory unless the sanctioned party can show that its violation of Rule 26(a) was either justified or harmless.” Rule text: Cornell LII.

Every circuit test verified for this page is a multi-factor harmlessness inquiry built on the same elements. The Third Circuit's Pennypack factors, applied in Opengate, are prejudice or surprise, ability to cure, disruption of trial, bad faith or willfulness, and the importance of the evidence. The Fourth Circuit's Southern States factors are similar, with the burden on the non-disclosing party. The Seventh Circuit's David v. Caterpillar factors, the Ninth Circuit's Lanard Toys factors applied in Isaac, and the Tenth Circuit's Woodworker's Supply factors applied in Spring Creek track the same list.

Exclusion is the default, not the only outcome. Isaac reopened limited depositions and shifted costs. McDavid struck part and allowed part. In Curd Minerals, LLC v. Diversified Production, LLC (S.D. W. Va. May 18, 2023), the court denied motions to strike late reports and a rebuttal expert, and ordered the parties to confer on reopening discovery and resetting trial. The working rule for a search expert: export the data on the date of the report, log what changed and when it became available, and serve the update as soon as it is in hand rather than at the outer deadline.

Frequently Asked Questions

Published 2026-11-04