An Expert Witness Guide to Supplemental Reports

By Noah Bolmer 

Supplemental reports give an expert a way to keep an existing report current as the factual record develops. New documents may affect a calculation, additional discovery may supply information that was previously unavailable, or an error may need to be corrected. The resulting report can be brief or extensive depending on what has changed, and the significance of the update can vary considerably from one case to another. A well-prepared supplement should make clear what prompted the additional work and how it relates to the analysis already provided. 

Background 

Federal Rule of Civil Procedure 26(e) establishes a continuing obligation to correct or supplement discovery disclosures when they become materially incomplete or incorrect. It provides: 

A party who has made a disclosure under Rule 26(a) […] must supplement or correct its disclosure or response […] in a timely manner if the party learns that in some material respect the disclosure or response is incomplete or incorrect. 

For expert reports prepared under Rule 26(a)(2)(B), that obligation specifically extends to information contained in the report and information provided during deposition. The rule further provides that additions or changes must generally be disclosed by the time pretrial disclosures are due. State rules, on the other hand, vary. Some closely track the federal approach, while others impose different requirements or deadlines. The applicable jurisdiction, scheduling order, and local rules therefore control in a particular case. The federal rule provides the framework for much of the case law concerning supplemental reports, but its terminology and requirements should not be assumed to apply identically in every court. 

The Rules do not prescribe a particular document called a “supplemental expert report” or establish a required format. The term commonly refers to a subsequent report or disclosure that adds to or corrects an earlier report. Courts have treated supplemental reports in a variety of circumstances, with the original report remaining the starting point for evaluating the later disclosure. 

What Belongs in a Supplemental Report 

There is no prescribed format for a supplemental report, and the appropriate approach depends on the nature and extent of the supplementation and the requirements of the applicable scheduling order. For a modest update, the supplement can be relatively simple. It can identify the material being added or corrected and present the resulting analysis without reproducing portions of the original report that remain unchanged. For a calculation, that might mean identifying a revised input and showing its effect on the result. When an error is corrected, the original calculation or statement can be identified alongside the correction. 

More extensive supplementation may call for a more structured comparison with the original report. The expert might identify the sections or opinions affected and indicate which portions remain unchanged. A table showing original and revised figures can be useful for a calculation-heavy report, while a redline or similar comparison may make substantial revisions easier to evaluate. None of these formats is generally required, but they can make a lengthy supplement easier to follow. The more extensive the changes, the more useful it becomes to give the reader a clear way to distinguish the new material from the portions of the original report that remain in place. 

The format should ultimately make the relationship between the original and supplemental reports easy to understand. A reader should not have to reconstruct that relationship from scratch, particularly when the supplement makes substantial revisions or addresses multiple parts of the original analysis. A concise explanation at the outset, followed by clearly identified changes and their supporting analysis, can accomplish that without turning the supplement into a second version of the original report. 

What Triggers a Supplement? 

Rule 26 establishes the duty to supplement when an expert disclosure becomes materially incomplete or incorrect. Courts have interpreted that duty through decisions addressing the circumstances in which a later disclosure is appropriate, including the distinction between information that becomes available after the original report and information that could have been obtained earlier. Those distinctions become important when determining whether a later disclosure properly supplements the original report. 

• Unforeseen information. Additional records, data, testimony, or other evidence may come to light after the original report that could not reasonably have been anticipated when the report was prepared. If new unforeseen information affects an existing opinion or the information underlying it, it may provide a basis for supplementation. The same principle can apply when a development occurs outside the expert’s control and changes the factual record on which the analysis depends. 

• Information expected to become available. Some analyses depend on information that is still being collected or developed when the initial report is due. An expert may supplement when that information later becomes available and materially affects the existing analysis. The distinction is between information that was genuinely unavailable at the time of the original report and information that could have been obtained and included then. The initial report still must provide a complete statement of the opinions to be offered, based on the information reasonably available at the time. 

• A material error discovered after the report. An expert may discover after submitting a report that a factual statement, calculation, citation, or other material part of the disclosure was incorrect. Once identified, an error that affects the accuracy or completeness of the disclosure may require correction through supplementation. The obligation can arise even when the underlying information was available when the original report was prepared, because the issue is the accuracy of what was actually disclosed. 

When a Supplemental Report Goes Too Far 

The difficulty arises when a change to the underlying information produces a change in the expert’s analysis. A revised result may follow directly from new inputs without changing the analysis that produced the original opinion. At some point, however, the later work can introduce a new opinion, methodology, or theory that belongs in the original report. 

Same Methodology, New Inputs 

In Sonrai Systems, LLC v. Romano, the court considered a 92-page supplemental report that substantially changed the damages calculations. Lost-profit damages increased from $45.8 million to $60.4 million, while the unjust-enrichment calculation fell from $43.7 million to $3.9 million. The court permitted the changes where newly permitted evidence affected the inputs to the existing analyses, while the underlying methodology remained the same. It struck other portions that relied on information already available when the original report was prepared or introduced new theories and categories of damages. The court ultimately required an amended report conforming to those limitations. 

Sales or Profits? 

Sacks Holdings, Inc. v. Grin Natural USA Ltd. involved a different kind of change. The initial report addressed sales and stated that additional information would be needed before profits could be calculated. A subsequent report calculated profits after the expert received additional financial information. The court held that the later report offered new opinions and calculations because the initial report addressed sales while the later report additionally addressed profits. The court also found that the necessary information had been available to the defendants before the original expert report deadline. It therefore rejected the characterization of the later report as a Rule 26(e) supplement and struck it. 

When “New” Evidence Isn’t New 

Wilkerson v. Carnival Corporation illustrates the problem with a purported supplement based on information that was already available. The plaintiff’s expert issued a supplemental report after the close of discovery, relying on arthroscopic photographs that had been available to the plaintiff and her counsel for more than 19 months but had not been provided to the expert for the original report. The court concluded that the report contained new substantive opinions based on that information and struck it under Rule 37(c)(1). 

More Data, Same Model 

Skibo v. Greer Laboratories, Inc. provides a useful counterpoint. The supplemental damages report incorporated additional sales information, documents, admissions, and customer testimony into the expert’s existing damages model. The methodology remained fundamentally unchanged, although the damages calculation increased from $52.9 million to $57.4 million. The court found the disclosure timely and concluded that exclusion was unwarranted because the disclosure was harmless and substantially justified. It denied the motion to strike and gave the opposing party 60 days to designate a rebuttal expert. 

Conclusion 

A supplemental report should preserve a clear connection to the disclosure that came before it. That does not mean every later change must produce the same opinion, calculation, or result, particularly when the underlying information has changed. It does mean that the reason for the change should be identifiable, the relationship to the original analysis should be clear, and the opposing party should be able to understand what has changed and why. 

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