Should You Share an Opposing Expert's Report With Your Expert?

The Strategic Question Behind an Opposing Expert Report

Whether to share an opposing expert witness report with your retained expert is usually a strategic sequencing decision, not a simple disclosure-rule question. Counsel may want an expert's unfiltered view of the evidence before the expert sees the other side's theory. In other cases, early review of the opposing analysis may reveal missing data, methodological weaknesses, or issues that require prompt attention.

The right approach depends on the case, the expert's discipline, the evidence available, the disclosure schedule, and any court-specific requirements. It may also depend on whether counsel needs an initial opinion, a focused critique, or assistance preparing for an expert deposition or trial.

A useful starting point is to distinguish two questions:

1. What must the party disclose to the opposing side under the governing rules and scheduling order?
2. When, if at all, should counsel provide the opposing report to its own retained expert?

The first is a legal-compliance issue. The second concerns how to obtain a reliable, well-supported expert analysis while preparing for the other side's opinions.

Disclosure Obligations and Strategic Sharing Are Different Questions

In federal civil litigation, Federal Rule of Civil Procedure 26(a)(2) addresses disclosures concerning expert testimony that a party may use at trial under Federal Rules of Evidence 702, 703, or 705. Rule 26(a)(2)(B) generally requires a written report prepared and signed by a retained expert when the rule applies, subject to stipulation or court order.

Those requirements govern disclosures between parties. They do not, by themselves, answer the separate workflow question of when counsel should show an opposing expert's report to a retained expert. Applicable obligations can vary with the governing law, local practice, scheduling order, type of expert, and posture of the case. Attorneys should review the operative rules and orders before setting an expert-review process.

For background on what an expert report must address, see general expert witness report requirements. The report's contents can help counsel identify what should be evaluated after the opposing analysis becomes available: the opinions offered, supporting facts or data, methodology, qualifications, assumptions, and exhibits.

The practical question is not whether an attorney can benefit from understanding the opposing report. In most contested expert matters, that benefit is clear. The harder question is whether review should occur before or after the retained expert has independently examined the relevant record, evidence, and technical issues.

Potential Risks of Early Exposure

Early exposure to an opposing opinion can influence how an expert frames the problem, weighs facts, or understands the available range of conclusions. That possibility does not mean the expert's opinion is necessarily compromised. It means counsel should consider whether the timing of review could affect the independence and clarity of the expert's analysis.

Confirmation Bias

Confirmation bias occurs when a person sees what they expect to see. Consider, for example, the case of Brandon Mayfield. The Federal Bureau of Investigation erroneously matched Brandon Mayfield's fingerprint to that of bomb-making materials found near a terrorist attack in Spain. In that case, both members of the FBI and an independent court expert declared the prints a “match” — a finding that was later proved false. According to the American Bar Association, the error was partially attributed to confirmation bias.

The Mayfield Study Discussion

To explore confirmation bias, Itiel E. Dror and others conducted an experiment involving five fingerprint examiners who had not seen the Mayfield prints. The examiners received a questioned print they were told was from the terrorist attack and a set of prints purportedly from Mayfield. They were then asked whether they saw a match.

By that point, the examiners knew the Mayfield matter involved a close non-match rather than an actual match. Unbeknownst to them, however, the prints were not from the Mayfield case. They came from the examiners' own case files, where the examiners had previously identified matches. Three of the five examiners concluded that the prints did not match, one found the information insufficient and reached an “inconclusive” result, and one retained the original match conclusion.

The study narrative illustrates a practical point for litigation teams: contextual information can affect professional judgment. An opposing report may provide useful context, but it can also become an anchoring point. The report's author may matter as well. A well-known expert's reputation, or an expert's prior view of that person, may influence how the report is received.

These concerns are factors to assess, not a rule against sharing an opposing report. Experienced experts may be fully capable of reviewing competing opinions critically. Still, an independent first look can provide counsel with a clearer record of the expert's own analysis before the competing theory enters the process.

Potential Benefits of Reviewing the Opposing Analysis

Reviewing an opposing report can be highly valuable once counsel has determined that the timing is appropriate. A competing analysis may identify an additional source of data, an alternative calculation, a factual assumption, or a methodology that deserves attention.

An expert's report typically identifies the materials reviewed and the path from those materials to the expert's conclusions. Comparing those inputs with the materials considered by your expert can reveal meaningful differences. One expert may have had access to more complete records, used a different data set, applied a different testing protocol, or relied on assumptions that the other expert did not share.

That review may lead an expert to refine an opinion after considering additional information. While a changed opinion can create difficult pretrial decisions, it is usually better to understand a relevant factual or methodological issue before cross-examination than to encounter it for the first time at trial.

Review can also help counsel prepare targeted discovery. If the opposing expert omitted information that your expert views as material, the issue may warrant further investigation or focused questioning in deposition. If the report uses a disputed methodology, your expert may help identify the questions needed to understand whether that methodology was applied consistently and reliably.

• Independent analysis: Questions to assess: Does counsel need an unanchored initial view?; Possible approach: Obtain an initial assessment before sharing the report.
• Competing methodology: Questions to assess: Does the opposing report identify different inputs or methods?; Possible approach: Ask for a focused methodology review.
• Schedule: Questions to assess: Is a disclosure, deposition, or report deadline approaching?; Possible approach: Align review timing with the case schedule.
• New evidence: Questions to assess: Does the report identify material not previously reviewed?; Possible approach: Provide the new material and document the timing.
• Court direction: Questions to assess: Does an order control disclosures or rebuttal timing?; Possible approach: Follow the order and seek legal guidance as needed.

A Sequenced Review Protocol

A blended approach can preserve the benefits of independent analysis while allowing a meaningful review of the opposing expert's work. In an appropriate matter, counsel may first ask the retained expert to assess the evidence and state preliminary opinions based on the materials then available. After that assessment, counsel can provide the opposing report for a targeted comparison.

This process is sometimes described as sequential unmasking. The goal is not to create a procedural stunt or to conceal the realities of litigation from the expert. Experts understand that another side may retain an expert with a different theory. The goal is to establish an independent starting point and then ask the expert to explain where, if anywhere, the opposing analysis changes the assessment.

Clear communication matters. Counsel can explain at the outset that the expert may be asked to conduct an initial review and later evaluate a competing report. An expert should understand the scope of each assignment, the questions being asked, and the materials being supplied at each stage.

A practical protocol may include:

• Defining the initial assignment and the factual or technical questions to be addressed.
• Maintaining an organized record of the materials provided at each stage.
• Identifying when the opposing report was received and when it was sent to the retained expert.
• Asking the expert to distinguish between an independent opinion and any later response to specific competing opinions.
• Considering whether newly identified evidence requires further analysis.
• Reviewing governing rules, court orders, and deadlines before requesting any revised or additional written opinion.

This documentation can improve case management and make it easier for counsel to understand the development of the expert's analysis. It can also support clear and independent expert communication throughout the matter.

When Review May Lead to a Rebuttal Report

An opposing report may reveal an issue that calls for a response, but review does not automatically require a rebuttal report. A rebuttal may be appropriate when a response is permitted and needed to address opinions, methods, assumptions, or evidence raised by the opposing side.

The scope and timing of any response depend on the applicable rule, scheduling order, and relationship between the proposed response and the opposing opinions. Counsel should avoid treating a rebuttal report as an opportunity to add opinions that should have appeared in an initial report.

For a discussion of scope and preparation considerations, see how to prepare an expert rebuttal report. A disciplined review process helps counsel decide whether the opposing report presents a genuine issue for rebuttal, a point for deposition, or simply a competing conclusion that the evidence will resolve at trial.