The Expert Witness Report

The document is written to stand on its own

A report served under FRCP 26(a)(2)(B) is not a summary, a letter to retaining counsel, or a preview of what the witness might say. The 1993 Advisory Committee Note describes it as a document “intended to set forth the substance of the direct examination,” and the first of the six required contents is “a complete statement of all opinions the witness will express and the basis and reasons for them.” Complete is the operative word, and it is the word most often treated as aspirational.

The sequencing rule makes the point structural. FRCP 26(b)(4)(A) provides that where a report is required, “the deposition may be conducted only after the report is provided.” The report is the fixed document; the deposition is the examination of it. An opinion the report never disclosed does not enter the case by being mentioned under oath four months later, because the later addition runs into the supplementation limits of FRCP 26(e) and the exclusion sanction in FRCP 37(c)(1).

Every drafting decision below follows from that. The question at each point is not whether the report reads well, but whether it would survive being the only account of the opinion that the court, the opposing expert and the cross-examiner ever get.

Who writes it, and what counsel may legitimately do

The rule requires a report “prepared and signed by the witness.” The accepted middle position on attorney involvement comes from the 1993 Advisory Committee Note, which is more permissive than the folklore suggests:

“Rule 26(a)(2)(B) does not preclude counsel from providing assistance to experts in preparing the reports, and indeed, with experts such as automobile mechanics, this assistance may be needed. Nevertheless, the report, which is intended to set forth the substance of the direct examination, should be written in a manner that reflects the testimony to be given by the witness and it must be signed by the witness.”

The line drawn there is between assistance and authorship. ABA Section of Litigation materials on working with experts put the practice standard bluntly: it is important that experts draft their own reports in their own style and format, reflecting their own opinions, although opinions may be discussed with the expert before the report is finalized.

The reason is not etiquette. A report whose voice, vocabulary and structure belong to the lawyer invites the single most damaging line available on cross-examination — who wrote this. A related failure is the expert who did not do the work: where the analysis was performed by staff and the named witness cannot explain it, the opinion is exposed under FRE 702(b) and 702(d) and under the rule's own requirement that the report be prepared by the witness.

Opinions stated one at a time, each with its own basis

The most common structural defect in a report is a narrative that describes an investigation and then arrives at a conclusion, leaving the reader to infer which facts support which opinion. That structure is comfortable to write and impossible to defend, because the deposition will take each opinion separately and ask what supports it, and the answer will have to be assembled on the spot.

A report built the other way — each opinion numbered, each followed by the documents, data, tests, observations and assumptions that support it, and each distinguishing what was tested from what was assumed — costs nothing extra and forecloses a line of examination. It also makes the report usable by the court on a Rule 702 motion, which is decided on the papers more often than at a hearing.

Two things belong inside that structure and are frequently left out. The first is the standard of certainty to which each opinion is stated, expressed in terms the method can actually support. The 2023 Advisory Committee Note to FRE 702 is explicit that experts “should avoid assertions of absolute or one hundred percent certainty — or to a reasonable degree of scientific certainty — if the methodology is subjective and thus potentially subject to error.” The second is the boundary: what the witness is not opining on. A report that says so in terms is far harder to stretch, in either direction, at trial.

The materials list records what was considered, not what was used

Item (ii) requires “the facts or data considered by the witness in forming” the opinions. The 2010 amendment replaced the older phrase “data or other information,” and the Advisory Committee explained both halves of the change:

“The refocus of disclosure on 'facts or data' is meant to limit disclosure to material of a factual nature by excluding theories or mental impressions of counsel. At the same time, the intention is that 'facts or data' be interpreted broadly to require disclosure of any material considered by the expert, from whatever source, that contains factual ingredients. The disclosure obligation extends to any facts or data 'considered' by the expert in forming the opinions to be expressed, not only those relied upon by the expert.”

That last clause is the one routinely written out of practitioner summaries, which describe the list as a list of what the expert relied on. It is not. A study read and rejected, a model run and discarded, a document from counsel reviewed and set aside — all three were considered. The drafters' own usage confirms it: the narrower verb “relied on” appears in the expert rules exactly once, at FRCP 26(b)(4)(C)(iii), for attorney-supplied assumptions, immediately after (C)(ii) uses “considered” for attorney-supplied facts. Two different words in adjacent subparagraphs are not synonyms.

The practical consequence is a file that can produce everything: what counsel sent, what the witness obtained independently, the data, the testing records, the calculations and the models.

Exhibits, and the line the 2024 rules drew through them

Item (iii) requires “any exhibits that will be used to summarize or support” the opinions — charts, models, animations, demonstratives. The report is where they are disclosed, and it is worth disclosing them knowing what they will later be sorted into.

Since 1 December 2024 the federal rules treat two things that used to travel together as separate categories. FRE 1006 covers a summary, chart or calculation offered to prove the content of voluminous admissible materials; it is admitted as evidence and it goes to the jury room. New FRE 107 covers an illustrative aid, which is not evidence and does not go back absent consent of all parties or a good-cause order. FRE 1006(c) routes anything functioning only as an aid to Rule 107.

For a report, that has a drafting consequence. A damages model built from voluminous admissible business records is a candidate for Rule 1006 treatment, and the underlying materials have to be made available to the other parties. A timeline of the litigation, an animation, or a slide restating the witness's own conclusions is an illustrative aid whatever it is labelled. A single exhibit that mixes summarized record data with the expert's characterizations is neither cleanly, and courts have handled the hybrid by admitting the record-derived portion, treating the argumentative portion as an aid, or refusing the exhibit as drawn.

Two lists, two different clocks

Items (iv) and (v) are the parts of the report most often described wrongly, including in materials written for experts. They run on different periods and cover different things.

Publications: ten years. The rule requires “the witness's qualifications, including a list of all publications authored in the previous 10 years.” It is not confined to peer-reviewed work and not confined to publications on the subject of the case. A blog post, a trade article, a book chapter and a conference paper are all publications the witness authored.

Prior testimony: four years. The rule requires “a list of all other cases in which, during the previous 4 years, the witness testified as an expert at trial or by deposition.” Three limits sit on the face of the text: four years rather than ten; testimony at trial or by deposition only, not affidavits, declarations or reports in matters that never reached testimony; and “cases,” which courts generally read to require enough identifying detail — case name, court, docket number, retaining party, and whether trial or deposition — for an opponent to find the transcript.

Writing “ten years of prior testimony” or “four years of publications” inverts the rule, and both errors appear in circulation. Whether the testimony list captures arbitrations, administrative hearings and sworn declarations is genuinely unsettled; the text says trial or deposition, and many experts over-disclose rather than litigate it.

The compensation statement, and what it is not

Item (vi) requires “a statement of the compensation to be paid for the study and testimony in the case.” What the rule asks for is the compensation arrangement for this engagement — the structure under which the study and the testimony are billed. It is a disclosure about this matter, not an account of the witness's practice.

The wider financial picture is not thereby off limits; it is obtained elsewhere. FRCP 26(b)(4)(C)(i) removes work-product protection from communications between counsel and the expert that “relate to compensation for the expert's study or testimony,” which makes the billing side of the engagement discoverable. Questions about annual volume of engagements, the balance between plaintiff and defense retentions, and the share of professional income derived from litigation work are put at deposition, where they belong.

That distinction matters for a second reason. Compensation evidence is bias material, not exclusion material. It supports cross-examination on credibility and, occasionally, an argument that a method was developed to serve the retention; it almost never supports exclusion under Rule 702. Treating the two as the same thing is one of the most common errors in writing aimed at practitioners.

What a good report does that the rule does not require

Compliance is the floor. The reports that survive contact tend to share a small number of features none of which appear in FRCP 26(a)(2)(B).

• They separate fact from assumption in terms. Every assumption is labelled as one, with its source identified — particularly where the source is counsel, since FRCP 26(b)(4)(C)(iii) makes attorney-supplied assumptions discoverable anyway.
• They say what was not available. Data never generated, a site never accessible, a device never produced. An acknowledged gap is a limit on the opinion; an unacknowledged one is an exhibit at deposition.
• They address the contrary material. A study that cuts the other way, discussed and answered, is a paragraph. The same study raised for the first time on cross is an event.
• They are reproducible. Another qualified person, given the same materials and the described method, should be able to follow the path to the conclusion. That is what FRE 702(c) and 702(d) are asking about.
• They stop where the expertise stops. Qualification is assessed opinion by opinion rather than witness by witness, and partial exclusion is a far more common outcome than wholesale exclusion.

None of that makes a report unassailable. It makes the report the strongest document in the case rather than the weakest, which is the realistic objective.

Frequently Asked Questions

Published 2026-08-12