Expert Witnesses in New York State and Federal Courts: Your Best Case Depends on Knowing the Difference
9.9.2026
Experts are a critical part of complex cases, whether in state or federal court. But for parties and practitioners, the timing and scope of expert disclosure, the form of the expert report and the standard for admissibility vary widely depending on whether the case is in New York Supreme Court, the Commercial Division or U.S. District Court. Understanding these differences before expert discovery is critical to putting on your best possible case.
Expert Disclosures
When a party intends to call an expert, the party must inform its opponent of the expert and their anticipated testimony. Federal courts require extensive expert reports well before trial. By contrast, New York courts generally permit more limited disclosure on a more flexible timeline. But the Commercial Division requires thorough and early disclosures, more like the federal scheme.
Federal Rules – Disclosure and Report: Federal Rule of Civil Procedure 26 requires any party relying on an expert witness to disclose both the expert’s identity and an expert report, as required by Rule 26(a)(2)(B).[1] By default, those disclosures must generally be made at least 90 days, although courts often require earlier disclosure. Any rebuttal expert report must by default be served within 30 days of the opening report. Those deadlines can be modified by stipulation or court order.
Federal expert reports must contain the following:
- A complete statement of all opinions the witness will express and the basis and reasons for them.
- The facts or data considered by the witness in forming them.
- Any exhibits that will be used to summarize or support them.
- The witness’s qualifications, including a list of all publications authored in the previous 10 years.
- A list of all other cases in which, during the previous four years, the witness testified as an expert at trial or by deposition.
- A statement of compensation to be paid for the study and testimony in the case.
The second requirement – that the report contain every fact or piece of data the expert considered – requires a particularly thorough production to opposing counsel.[2] Courts broadly construe “facts or data considered” to cover anything “received, reviewed, read or authored by the expert, before or in connection with the forming of their opinion, if the subject matter relates to the facts or opinions expressed.”[3] This includes nearly all material the expert considered in formulating their opinion, not just what ultimately relied on.
The expert, not counsel, must “prepare[] and sign[]” the expert report.[4] Although attorneys may assist in drafting, the report must reflect the expert’s opinions. This requirement has important implications for the discoverability of draft reports. In general, Federal Rule 26(b)(4) protects communications between the expert and counsel, including draft reports. However, communications that identify facts or data that counsel provided and the expert considered (not just relied upon) are not protected. Communications about compensation and about assumptions the attorneys provided and the expert relied upon also are not protected.
CPLR – Disclosure Upon Request: For practitioners accustomed to the relatively specific disclosures required by the Federal Rules of Civil Procedure, the obligations under the New York Civil Practice Law and Rules (CPLR) are quite different. Unlike in federal court, a party need produce an expert disclosure only “[u]pon request from opposing counsel.[5] And the timeline is more flexible. Generally, expert disclosure does not occur until after the close of discovery. Some courts have admitted testimony from experts first disclosed at trial where the late disclosure caused no prejudice.[6]
The disclosures themselves are more limited, too. They need only contain:
- The subject matter on which each expert is expected to testify.
- The substance of the facts and opinions on which each expert is expected to testify.
- The qualifications of each expert witness and a summary of the grounds for each expert’s opinion.
Critically, the CPLR requires disclosure only of the topics the expert will address; not all facts and data considered.
Nor must experts in New York State court prepare the report themselves. Attorneys can, and often do, prepare the statement.[7] Yet, paradoxically, the work-product protections in New York State are more amorphous. There is no clear protection for communications between attorneys and testifying experts as there is in federal court, unless the communications constitute the attorney’s own work product.[8] Parties in New York Supreme Court should therefore consider a stipulation specifying which communications will be protected from disclosure.
Commercial Division – More Like Federal Court: New York State practitioners should be wary, though. The Commercial Division has specialized rules, including expert-disclosure requirements, that take precedence over conflicting CPLR provisions.[9] Those requirements look more like the federal regime than the CPLR. Expert disclosures are mandatory, regardless of whether an opposing party requests them. Unlike in New York State Supreme Court, disclosures are due within four months after the close of discovery, and before the case is ready for trial. They also must include a report containing, among other things, “the data or other information considered by the expert in forming the opinions.” This mirrors the broad federal disclosure requirement.
Expert Depositions
New York State and federal law also diverge on expert depositions. In federal court, expert depositions are common. In most New York State courts, expert depositions are disfavored. The Commercial Division is yet again more like federal court; its rules are friendlier toward expert depositions.
Expert Depositions Under the Federal Rules: Federal Rule of Civil Procedure 26 allows parties to depose “any person who has been identified as an expert whose opinions may be presented at trial.”[10] An expert is thus treated like any other fact witness. If the deposition is relevant to the case and is not overly burdensome, the opposing party can depose the expert.
Expert Depositions Under the CPLR: Expert depositions are far less common in New York State Supreme Court. As a default rule, the CPLR does not permit a party to depose an expert. Courts may order an expert deposition only upon a showing of “special circumstances.”[11] This requirement is strictly construed. The party seeking the deposition must show why a deposition, rather than another form of discovery, is necessary.[12]
Expert depositions may soon become more widely available in commercial cases. In 2025, the recommended bifurcating the statute in its report to the Chief Administrative Judge of the Courts of the State of New York.[13] In its view, the special-circumstances exception could continue to apply generally, but parties should no longer have to show special circumstances to secure expert depositions in commercial cases worth over $250,000. In those cases, the party requesting a deposition would need to show only that its need for the deposition “outweighs the . . . expense and delay” of conducting it.
Expert Depositions Under the Commercial Division Rules: The Commercial Division breaks with the approach of other New York State Supreme Courts. Parties there must agree on a schedule for depositions of testifying experts within 30 days after the close of fact discovery.[14] No finding of special circumstances is necessary. In the Commercial Division, expert depositions, like expert reports, must be completed no later than four months after the close of fact discovery.
Admissibility of Expert Testimony
Perhaps the most significant difference between federal and state expert practice is the standard governing admissibility. Federal courts have broad discretion to exclude unreliable expert testimony. New York State courts defer to the consensus of others in the relevant field.
Federal Rule of Evidence 702: Federal Rule of Evidence 702 governs the admissibility of expert testimony in federal court (often called the Daubert standard, after the U.S. Supreme Court case that presaged modern rule). Preliminarily, all experts must be “qualified” by “knowledge, skill, experience, training, or education.” Courts assess a witness’s experience as a whole, rather than viewing individual qualifications in isolation.[15]
A qualified expert can offer opinion testimony under Rule 702 when four conditions are met:
- The expert’s scientific, technical or other specialized knowledge will help the trier of fact to understand the evidence or to determine a fact in issue.
- The testimony is based on sufficient facts or data.
- The testimony is the product of reliable principles and methods.
- The expert’s opinion reflects a reliable application of the principles and methods to the facts of the case.
Those provisions assess twin qualities of the expert’s testimony: reliability and relevancy.[16]
Federal judges have broad authority to determine whether an expert’s testimony is reliable. To determine whether an expert’s opinion is reliable under Rule 702, federal courts assess (1) whether an expert’s method “can be (and has been) tested”; (2) whether the method “has been subjected to peer review”; (3) the method’s “potential rate of error”; and (4) “the existence and maintenance of standards controlling the technique’s operation.”[17]
These factors incorporate some deference to professionals in the relevant field, but that deference is not absolute. Courts can, and do, override accepted methods based on perceived flaws in error rate. Considering the broad powers conferred by Rule 702, many authorities describe federal courts as “gatekeepers” of expert testimony.[18]
New York’s Frye Standard: New York state courts have a more limited role. Specifically, New York law instead entrusts reliability determinations to other experts in the field, rather than to judges.
The New York test for reliability is called the Frye test. Federal courts followed Frye before Rule 702, and several states still adhere to some variation of Frye.[19] Under Frye, expert testimony based on novel scientific evidence is admissible only if it is based on “general[ly] accept[ed]” procedures and principles.[20] The proponent has the burden of showing a scientific “consensus.”[21] This can be shown through scientific or legal writings, prior caselaw, or other expert opinions.[22] Although “consensus” is not clearly defined, the Court of Appeals has made clear that the scientific community need not unanimously endorse the method.[23] Some lower courts have even held a bare majority is enough.[24]
Frye is also narrower than Daubert. It only applies to evidence that is both novel and scientific in nature. Thus, if an expert relies on personal or non-scientific evidence, the Frye standard does not apply. And while Rule 702 hearings are common in federal court, a Frye hearing is appropriate only to resolve factual disputes about the relevant scientific consensus, not whether an expert has satisfied Frye.[25]
Outside of reliability, many aspects of the admissibility inquiry are the same. As in federal court, the testimony must be relevant and helpful to the trier of fact. And an expert must be qualified through “skill, training, education, knowledge or experience.” However, in state court, qualification determinations tend to go toward weight, rather than admissibility.[26] As a practical matter, expert testimony is far less likely to be excluded in New York State court.
Admissibility is one of the areas where the Commercial Division aligns with other New York trial courts and diverges from federal practice. Frye, not Rule 702, applies in all New York State fora.[27]
Reliance on Hearsay
Both federal and state law allow experts to rely on hearsay in forming opinions. In some instances, that hearsay can even be admissible. Federal law tends to be friendlier toward experts on both scores.
Federal Rule of Evidence 703: Rule 703 spells out what evidence an expert may rely on to form an opinion. Experts may always base their opinions on facts or data they have been made aware of or personally observed.[28] And experts can sometimes base opinions on facts and data outside of their personal knowledge – facts and data that would be excluded as hearsay if offered for their truth. An expert may rely on hearsay to form opinions if others in the field commonly rely on it. In other words, an expert’s opinion may be admissible, despite resting on inadmissible bases, if the expert followed common practice in the field.
Rule 703 also addresses when the underlying hearsay itself is admissible. Hearsay evidence underlying an expert opinion can only be disclosed to the jury if its probative value “substantially outweighs” the risk of it prejudicing the jury. In effect, Rule 703 creates a presumption against admitting hearsay evidence on which experts relied.
New York’s Professional Reliability Exception: Under New York law, an expert may always base his opinion on facts and data personally observed or that were otherwise admitted into the record.[29] An expert can also rely on evidence subject to the “professional reliability exception.” The exception has three elements. First, like under Federal Rule 703, the hearsay evidence must be ordinarily relied upon by other experts in the field; second, independent evidence must establish the hearsay evidence is reliable; and third, the hearsay evidence cannot be the primary basis for the expert’s opinion.
Whether such hearsay evidence is itself admissible is an open question. The Court of Appeals has recognized that there must be some limits on admitting inadmissible materials on which an expert relied.[30] Otherwise, expert testimony could become a conduit for hearsay. However, appellate courts are split on how to set those limits. The First and Second Departments have held that an expert can never present hearsay data to a jury. The Third Department has held that such data is admissible to help the jury understand the expert’s opinion, but not for its truth.[31]
Takeaways
Consider the standard for admissibility: When considering potential experts, counsel should think carefully about the standard the court will apply to scrutinize the expert’s opinions. The standard for admissibility – particularly where the expert’s opinions are based on their own experience – is radically different in federal court from the Commercial Division or New York State Supreme Court. That may affect both expert selection and the scope of opinions the expert may permissibly offer.
Recognize pre-trial disclosure varies wildly: Both federal courts and the Commercial Division require detailed disclosure of an expert’s opinions and permit the opposing party to depose those experts. Parties in a New York State court neither bear the burden of making those disclosures, nor get the benefit of receiving them from their opponents. This lack of disclosure may affect the scope of expert opinions, the timing of expert retention and trial preparation.
Prepare your expert: Relatedly, careful expert preparation can prevent many admissibility challenges. An expert familiar with the applicable admissibility standard will be more likely to produce admissible testimony. Preparation is particularly important in federal court, where experts are responsible for drafting their own reports. But even where counsel may assist in drafting, asking the expert to draft their own report can be advantageous. A jury may be more likely to credit language that an expert can explain and defend.
Negotiate stipulations: Rather than risking potentially onerous and occasionally unclear rules, parties can stipulate to expert procedures. A stipulation can establish mutually applicable rules governing, for example, what each side must disclose (such as materials “relied upon” versus “considered”), when disclosures must be made, whether and when underlying data must be disclosed, and when and how many expert depositions each side may take. Those stipulations provide certainty – a positive in any jurisdiction.
Lauren F. Dayton is a partner at MoloLamken. She litigates commercial disputes and antitrust matters, frequently in New York state and federal courts.
Ryan M. Baldwin is an associate at MoloLamken. He focuses on complex business disputes and appeals.
Endnotes:
[1] Fed. R. Civ. P. 26(a)(2).
[2] Fed. R. Civ. P. 26, Adv. Comm. Notes, 2010 amend.
[3] In re Mirena IUD Prods. Liab. Litig., 169 F. Supp. 3d 396, 470 (S.D.N.Y. 2016).
[4] Fed. R. Civ. P. 26(a)(2)(B).
[5] N.Y. Civil Practice Law and Rules §3101(d)(1).
[6] Rivers v. Birnbaum, 102 A.D.3d 26, 38-39, 953 N.Y.S.2d 232, 240–41 (2d Dep’t 2012).
[7] See Veneski v. Queens Long Is. Med. Grp., 285 A.D.2d 369, 270, 727 N.Y.S.2d 105, 106 (1st Dep’t 2001).
[8] CPLR 3101(c). But see Hudson Ins. Co. v. Oppenheim, 72 A.D.3d 489, 899 N.Y.S.2d 29 (1st Dep’t 2010) (documents generated by consulting expert protected by attorney work product privilege).
[9] N.Y. Comp. Codes R. & Regs. tit. 22, §202.70(g).
[10] Fed. R. Civ. P. 26(b).
[11] CPLR 3101(d)(1)(iii).
[12] Connors, Practice Commentaries, McKinney’s Cons. Laws of NY, Book 7B, C:3101:29A.
[13]Report of the Advisory Committee on Civil Practice 81 (2025), https://www.nycourts.gov/LegacyPDFS/IP/judiciaryslegislative/pdfs/2025%20CPLR%20Committee%20Report.pdf.
[14] N.Y.C.R.R. tit. 22, § 202.70(g).
[15] Bee v. Novartis Pharms. Corp., 18 F. Supp. 3d 268, 300–01 (E.D.N.Y. 2014).
[16] Dependable Sales & Serv., Inc. v. TrueCar, Inc., 311 F. Supp. 3d 653, 658 (S.D.N.Y. 2018).
[17] Daubert v. Merrell Dow Pharms., Inc., 509 U.S. 579, 593–94 (1993).
[18] 29 Wright & Miller, Federal Practice & Procedure, §6266 (2d ed. 2026 update); see, e.g., United States v. Shipp, 422 F. Supp. 3d 762, 775 (E.D.N.Y. 2019).
[19] See, e.g., Ill. Evid. R. 702; Donaldson v. Cent. Ill. Pub. Serv. Co., 767 N.E.2d 314 (Ill. 2002).
[20] People v. Wesley, 633 N.E.2d 451, 454 (N.Y. 1994) (quoting Frye v. United States, 293 F. 1013, 1014 (D.C. Cir. 1923)).
[21] Sean R. ex rel. Debra R. v. BMW of N. Am., LLC, 26 N.Y.3d 801, 808–09, 28 N.Y.S.3d 656, 660 (2016).
[22] Cornell v. 360 W. 51st St. Realty, LLC, 22 N.Y.3d 762, 779–785, 986 N.Y.S.2d 389, 401–06 (2014).
[23] Sean R., 26 N.Y.3d at 808.
[24] Parker v. Mobil Oil Corp., 7 N.Y.3d 434, 446–47, 824 N.Y.S.2d 584, 589 (2006).
[25] People v. Oddone, 22 N.Y.3d 369, 375–76, 980 N.Y.S.2d 912, 915 (N.Y. 2013).
[26] Ghazala v. Shore Haven Apartment Del, LLC, 229 A.D.3d 447, 449, 214 N.Y.S.3d 747, 750 (2d Dep’t 2024).
[27] MBIA Ins. Corp. v. Countrywide Home Loans, Inc., 30 Misc. 3d 1201(A), 958 N.Y.S.2d 647 (N.Y. Sup. 2010).
[28] Fed. R. Evid. 703.
[29] Hambsch v. New York City Transit Auth., 63 N.Y.2d 723, 726, 480 N.Y.S.2d 195 (1984).
[30] Hinlicky v. Dreyfuss, 6 N.Y.3d 636, 648, 815 N.Y.S.2d 908 (2006).
[31] Compare Straus v. Strauss, 136 A.D.3d 419, 420, 24 N.Y.S.3d 76, 77 (1st Dep’t 2016) (citing Wagman v. Bradshaw, 292 A.D.2d 84, 739 N.Y.S.2d 421, 423-22 (2d Dep’t 2002)), with State v. Mark S., 87 A.D.3d 73, 78–79, 924 N.Y.S.2d 661, 665 (3d Dep’t 2011).
