What a Rebuttal Report Can and Cannot Do

The procedural position, and the deadline that usually is not the rule's

FRCP 26(a)(2)(D) sets the timing. Disclosures are made "at the times and in the sequence that the court orders," and absent a stipulation or court order they are due at least 90 days before trial — or, under subsection (ii), "if the evidence is intended solely to contradict or rebut evidence on the same subject matter identified by another party under Rule 26(a)(2)(B) or (C), within 30 days after the other party's disclosure."

Read the first clause before the second. The 90-day and 30-day structure applies only where nothing else has been set, and in practice a scheduling order nearly always sets its own expert and rebuttal dates. It is not correct to say that rebuttal reports are due 30 days after the opening report. The correct statement is that the rule's default is 30 days and the scheduling order controls — which is why the scheduling order belongs in the first email to any expert being considered for a rebuttal engagement.

The practical consequence is compression. A rebuttal expert is frequently working against a fixed date that was set before anyone knew what the opening report would contain, with a production that was assembled for a different purpose.

A rebuttal witness still files a full report

Nothing in the rule creates a lighter obligation for a responding expert. A retained rebuttal witness files a report under FRCP 26(a)(2)(B) containing the same six items as any other: a complete statement of all opinions with their basis and reasons, the facts or data considered, the exhibits to be used, the qualifications including the 10-year publication list, the 4-year list of cases in which the witness testified at trial or by deposition, and the compensation statement.

Two of those deserve attention in a rebuttal specifically. "The facts or data considered" now includes the opening expert's report and reliance materials, and it should include the underlying records the rebuttal actually reworked — which in this field usually means platform exports pulled independently rather than the opposing expert's derived tables. And the exhibits requirement rewards reproduction: a rebuttal exhibit that shows the opening expert's own number recomputed under the opening expert's own stated method is the most economical thing this kind of report contains.

The compensation and publication requirements catch people out at the same rate they do in an opening engagement. There is no shortcut because the engagement is responsive.

Same subject matter is the operative constraint

The phrase doing the work is "solely to contradict or rebut evidence on the same subject matter." It is a real limitation, and it is read as one: a rebuttal report responds to the other side's expert, and it is not a vehicle for the affirmative case a party failed to disclose on time.

A statement of candor is appropriate here. The rule text above is verified against the rule. The application described in the next two sections is the widely applied reading of it, and it is set out on this site as reasoning rather than as case law — no decision striking a rebuttal report for exceeding "the same subject matter" is cited here, because none was located in the research behind this page. Counsel evaluating exposure on this point in a particular forum should have authority from that forum rather than a general description.

What that means in practice is that the scope question gets settled before the work starts, not after. The subject matter of the opening report defines the perimeter, and an expert who wants to say something outside it should raise that with counsel as a disclosure question rather than write it into the report and let the motion sort it out.

What a rebuttal can do

Inside the perimeter, the available moves are more substantial than they first look.

• Identify errors in the opening expert's data, arithmetic or method. The ordinary case, and often the shortest section of the report.
• Show that the opening expert used the wrong definition of a metric. This is the single most productive rebuttal in this field, for a structural reason covered below.
• Show that the conclusion does not follow from the records relied on. A gap between what the exhibits contain and what the opinion asserts is a rebuttal finding, not an argument.
• Show that records the opening expert did not examine contradict the opinion, where those records bear on the same subject matter.
• Perform the reconciliation the opening expert should have performed and did not — for example, comparing what the platform charged against what the account's reporting shows against what the client was invoiced, where the opening report used only one of the three.
• State that the opening opinion is unsupported without offering an alternative quantification. This is a complete opinion and a sufficient one.

Note what unites the first five. Each is anchored to a record the opposing expert either used or should have used. A rebuttal built on records is difficult to answer; a rebuttal built on disagreement about judgment is a second opinion, and the jury has no way to choose between them.

What a rebuttal generally cannot do

The constraints follow from the same phrase.

• Introduce a new affirmative theory of liability, or a new damages model the party could have disclosed in its opening report.
• Open a subject matter the opening report did not address. If the opening expert opined only on spend reconciliation, a rebuttal that ranges into causation for a traffic decline is opening a new subject.
• Serve as a vehicle for an untimely opening opinion by labeling it rebuttal.
• Cure the party's own missed disclosure deadline.

The temptation is real, particularly on the defense side, where the opening report often reveals for the first time which theory has to be met and where an affirmative response looks obviously useful. The way to handle it is to raise the question with counsel while there is still time to seek leave or a stipulation, rather than to write the affirmative opinion into a document labeled rebuttal and hope the label holds. The downside is asymmetric: an excluded rebuttal leaves the opening opinion standing unanswered.

Why metric definitions make this field unusually productive for rebuttal

Digital marketing has an unusual number of metrics that are similar, adjacent, and easy to mix up — and the mixing produces numbers that are wrong by large multiples rather than by rounding.

The recurring set: "conversions" against "all conversions," which may include view-through; a figure computed under one attribution model against the same figure under another, where a report crossing September 2023 for a Google Ads account or November 2023 for an Analytics 4 property may straddle a forced model migration; a conversion window of one length against another, where Meta's available windows changed on 19 January 2021; served impressions against viewable impressions; the account time zone against the reporting time zone; a metric that includes modeled conversions against one that does not.

An opening expert who computes return on ad spend, cost per acquisition or "conversions" without stating the attribution model, the conversion window and which conversion actions were included has produced a number nobody can reproduce, including themselves. A rebuttal that pulls the platform export, applies the opening expert's own stated method, and shows the number moving is entirely record-based and needs no competing judgment at all. It is also, usefully, the kind of exhibit a court can follow without adopting anyone's expertise.

The one-source problem, and the records nobody opened

Opening reports in agency and vendor disputes are frequently built on a single record — most often the client's own copy of the agency's reporting, or one platform export produced early. That is a structural weakness the rebuttal position is well placed to exploit, because the second and third independent records usually still exist.

The platform's own billing record is separate from the reporting interface's cost column and is held by a party with no stake in either side's characterization. The change history shows what was configured, when, and under which login. The conversion-action configuration shows which model and window produced the numbers in dispute. The tag or container version history shows when measurement itself changed. None of those is exotic, and an opening report that used none of them has left the field open.

The discipline is the same one that applies to an opening analysis. Divergences between two records fall into three classes — definitional, timing, and unexplained — and a rebuttal that jumps to the third without eliminating the first two in writing has made the mistake it is criticizing. Platform figures for recent periods restate after the fact, and a report run on the 1st does not match the same report run on the 30th.

The causation gap as a rebuttal target

The most common defect in an opening report in this field is not arithmetic. It is a missing step: the report moves from "the defendant did X" to "the plaintiff lost $Y" without an intervening, record-based link between them.

The rebuttal of that gap does not require an alternative number. It requires showing which competing explanations the opening report never addressed and whether the records the opening expert had would have addressed them: whether the decline sits inside the range of the same weeks in prior years; whether other channels moved in the same shape at the same time; whether the onset falls inside a documented search ranking update rollout; whether a price change, a site migration, a tag change, a competitor entry or a platform definitional change sits in the same window. Each of those is checkable, and each one the opening report skipped is a live alternative it did not exclude.

Courts have excluded damages opinions in adjacent fields on exactly this ground. In Concord Boat Corp. v. Brunswick Corp., 207 F.3d 1039 (8th Cir. 2000), the Eighth Circuit held a model excludable because it "ignored inconvenient evidence" in the record. The transferable point for a rebuttal is that identifying the inconvenient evidence the opening expert passed over is itself the work.

The discipline that keeps a rebuttal inside its scope

There is one temptation that ends more rebuttal engagements badly than any other, and it arrives late in the drafting.

Having shown that the opening expert's number is unreproducible, the expert reaches for the sentence "and therefore the correct figure is Z." That sentence changes what the report is. An expert who computes Z without having been asked to compute Z has stopped rebutting and started offering an affirmative opinion, which may be untimely, may be outside the scope, and in any event carries a burden the engagement was never resourced to meet.

"This analysis does not support this conclusion, and here is why" is a complete and sufficient rebuttal opinion. It is also, in most of these matters, the accurate one — because the reason the opening number is unreproducible is usually that the underlying records do not determine a single figure at all.

Two related limits belong in the report rather than in cross-examination. A rebuttal is bounded by the production it was given, which was assembled for someone else's purposes; saying what was not available is not a weakness. And a rebuttal that identifies errors in an opinion has said something about that opinion, not about the merits of the claim. Those are different statements and a careful report keeps them apart.

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Published 2026-08-12