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Federal Rules of Civil Procedure
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Expert declarations can create exclusion risks when they go beyond proper Rule 26(e) supplementation and introduce new opinions, methodologies, or reasoning after disclosure deadlines. This article explains how courts distinguish permissible clarification from improper expansion, how Rule 37(c)(1) applies, and the steps attorneys can take to reduce the risk of late expert testimony being limited or excluded.
Expert testimony often plays a central role in cases involving technical or specialized issues, especially at summary judgment and trial. When expert opinions are excluded, the consequences can be significant and may determine whether a claim survives.
Courts have made clear that inadequate expert disclosures can lead to exclusion, which in turn can affect a party’s ability to prove its case. Decisions such as Salgado v. General Motors Corp. show how insufficient expert reports can result in the loss of testimony that is critical to a claim.
A recurring issue in litigation is whether an expert declaration goes beyond what was originally disclosed. This often arises when a party submits a declaration that appears to expand on, revise, or strengthen opinions after the expert report or deposition.
Opposing counsel may challenge that declaration as an improper attempt to introduce new opinions after disclosure deadlines have passed.
Courts take these disputes seriously. Disclosure rules are designed to prevent surprise and give both sides a fair opportunity to prepare. When new expert opinions appear late in the process, the opposing party may have limited ability to test or respond to them. Courts address these concerns through Rule 37(c)(1), as reflected in cases such as Yeti by Molly, Ltd. v. Deckers Outdoor Corp.
Expert disclosures in federal court are governed by Rule 26(a)(2) of the Federal Rules of Civil Procedure. This rule requires a written report containing a complete statement of all opinions the expert will express, along with the basis and reasons for those opinions, the data considered, and the expert’s methodology.
In practice, the expert report sets the foundation and boundaries of the expert’s testimony.
Rule 26(e) imposes a duty to supplement or correct disclosures and expert reports when they become materially incomplete or incorrect and the additional or corrective information has not otherwise been made known during discovery or in writing.
Courts draw an important distinction here. Proper supplementation is intended to correct or complete prior disclosures when they become materially incomplete or incorrect. It is not a mechanism to revise opinions, strengthen analysis, or introduce opinions that should have been included in the original report. Whether supplementation is permitted often depends on the nature of the change, the timing, and whether the opposing party is prejudiced.
Rule 37(c)(1) provides the enforcement mechanism. If a party fails to disclose information required by Rule 26(a) or Rule 26(e), the court may exclude that information or testimony unless the failure was substantially justified or harmless.
In determining whether exclusion or other sanctions are appropriate, courts often consider factors such as prejudice to the opposing party, the ability to cure the issue, potential disruption to the case schedule, and the explanation for the delay, although the precise formulation of these factors may vary by circuit.
These principles are reinforced in Salgado v. General Motors Corp., where the court upheld exclusion of expert testimony because the report failed to adequately disclose the basis, reasoning, and methodology required under Rule 26(a)(2).
Expert testimony often plays a central role in cases involving technical or specialized issues, especially at summary judgment and trial. When expert opinions are excluded, the consequences can be significant and may determine whether a claim survives.
Courts have made clear that inadequate expert disclosures can lead to exclusion, which in turn can affect a party’s ability to prove its case. Decisions such as Salgado v. General Motors Corp. show how insufficient expert reports can result in the loss of testimony that is critical to a claim.
A recurring issue in litigation is whether an expert declaration goes beyond what was originally disclosed. This often arises when a party submits a declaration that appears to expand on, revise, or strengthen opinions after the expert report or deposition.
Opposing counsel may challenge that declaration as an improper attempt to introduce new opinions after disclosure deadlines have passed.
Courts take these disputes seriously. Disclosure rules are designed to prevent surprise and give both sides a fair opportunity to prepare. When new expert opinions appear late in the process, the opposing party may have limited ability to test or respond to them. Courts address these concerns through Rule 37(c)(1), as reflected in cases such as Yeti by Molly, Ltd. v. Deckers Outdoor Corp.
Expert disclosures in federal court are governed by Rule 26(a)(2) of the Federal Rules of Civil Procedure. This rule requires a written report containing a complete statement of all opinions the expert will express, along with the basis and reasons for those opinions, the data considered, and the expert’s methodology.
In practice, the expert report sets the foundation and boundaries of the expert’s testimony.
Rule 26(e) imposes a duty to supplement or correct disclosures and expert reports when they become materially incomplete or incorrect and the additional or corrective information has not otherwise been made known during discovery or in writing.
Courts draw an important distinction here. Proper supplementation is intended to correct or complete prior disclosures when they become materially incomplete or incorrect. It is not a mechanism to revise opinions, strengthen analysis, or introduce opinions that should have been included in the original report. Whether supplementation is permitted often depends on the nature of the change, the timing, and whether the opposing party is prejudiced.
Rule 37(c)(1) provides the enforcement mechanism. If a party fails to disclose information required by Rule 26(a) or Rule 26(e), the court may exclude that information or testimony unless the failure was substantially justified or harmless.
In determining whether exclusion or other sanctions are appropriate, courts often consider factors such as prejudice to the opposing party, the ability to cure the issue, potential disruption to the case schedule, and the explanation for the delay, although the precise formulation of these factors may vary by circuit.
These principles are reinforced in Salgado v. General Motors Corp., where the court upheld exclusion of expert testimony because the report failed to adequately disclose the basis, reasoning, and methodology required under Rule 26(a)(2).
The label attached to a declaration does not control the analysis. Courts focus on what the declaration actually does.
A declaration may be considered improper supplementation when it introduces opinions, reasoning, methodologies, or substantive analysis that were not fairly disclosed in the original expert report. Courts commonly explain that Rule 26(e) is not intended to provide an opportunity to cure deficiencies caused by incomplete preparation or to extend expert disclosure deadlines.
Common examples of improper supplementation include:
At the same time, not every addition will lead to exclusion. Courts evaluate these issues in context.
For example, an expert may provide additional explanation supporting an already disclosed opinion. Courts generally examine whether that explanation was fairly disclosed in substance in the original report and whether the opposing party had notice of the expert’s reasoning.
If the later declaration introduces materially different reasoning, relies on analysis that was not fairly disclosed in the original report, or advances a new theory, courts are more likely to treat it as an untimely new opinion rather than a permissible clarification.
Late changes to expert opinions can create real challenges in litigation.
They can limit the opposing party’s ability to prepare, disrupt discovery deadlines, and require additional motion practice. These concerns are central to how courts apply Rule 37(c)(1).
In Yeti by Molly, Ltd. v. Deckers Outdoor Corp., the Ninth Circuit affirmed exclusion of an expert damages report that was not properly disclosed during discovery. The court explained that exclusion under Rule 37(c)(1) operates as a self-executing sanction in the sense that a separate motion to compel is not required before exclusion may be imposed. At the same time, courts retain discretion to determine whether the failure was substantially justified or harmless.
Courts continue to apply that framework in later cases. In Goodman v. Staples The Office Superstore, LLC, the Ninth Circuit affirmed limitations on expert testimony where disclosure deficiencies prevented the opposing party from fairly addressing the expert’s opinions.
Exclusion is not automatic in every case. Courts retain discretion under Rule 37(c)(1) to determine whether a disclosure failure was substantially justified or harmless. Depending on the circumstances, courts may consider alternatives such as reopening discovery, allowing additional depositions, or limiting the scope of testimony.
Courts typically compare the expert’s original report, deposition testimony, and the later declaration side by side.
A declaration is more likely to be excluded when it:
When a declaration materially contradicts prior testimony, courts may also evaluate whether it improperly attempts to create factual disputes through inconsistent statements.
On the other hand, courts may allow additional explanation when it remains grounded in the original report. This can include elaborating on reasoning already disclosed or responding to criticisms raised by the opposing party, as long as the core opinions remain the same.
The key question is whether the opposing party had a fair opportunity to understand and respond to the expert’s opinions.
This issue often becomes critical at summary judgment, where expert declarations can determine whether a claim proceeds.
Because expert testimony is often necessary to establish elements such as causation or damages, exclusion at this stage can significantly affect case outcomes.
After a deposition, experts sometimes attempt to clarify or refine their opinions.
Courts often reject declarations that materially change prior testimony, especially when they appear to address weaknesses exposed during deposition. Limited clarification may be allowed if it remains consistent with the original report and testimony.
Parties may argue that a declaration is proper rebuttal, but courts focus on how the declaration functions.
Rebuttal is generally limited to addressing or contradicting the opposing expert’s opinions, rather than advancing new affirmative opinions that could reasonably have been disclosed earlier in the party’s case-in-chief.
Courts examine whether the declaration stays within the scope of responding to the opposing expert or instead expands beyond it.
Many state courts apply similar principles, although the specific rules and procedures vary.
For example, California’s expert exchange framework under Code of Civil Procedure provisions governing expert witness information exchange includes identification of expert witnesses, procedures for exchanging expert information, expert declarations in certain circumstances, deposition rights, and limitations on testimony beyond the scope of disclosed subject matter.
Across jurisdictions, courts focus on the same core concern: whether the opposing party had a fair opportunity to prepare for and respond to the expert’s opinions.
In practice, the safest approach is to treat the expert report as the foundation of the case.
To the extent reasonably foreseeable at the time of disclosure, the report should include the expert’s intended opinions, supporting analysis, methodology, and material assumptions. Courts may exclude testimony when reports fail to adequately explain the expert’s reasoning or methodology.
Depositions provide an opportunity to confirm the expert’s reasoning and address any ambiguity.
Focus on clarifying assumptions, testing the analysis, and ensuring that the expert’s opinions are fully developed on the record. Gaps left at this stage can be difficult to address later.
When preparing a declaration, stay grounded in the original disclosures.
A useful question is whether the declaration explains what is already in the record or introduces something new. If it introduces new substance, it may create a risk of exclusion under Rule 37(c)(1).
If the opposing party submits a questionable declaration, act promptly.
Compare the declaration to the expert report and deposition testimony. Identify any new opinions or inconsistencies and consider whether a motion to exclude is appropriate.
If a court finds that a declaration improperly supplements an expert opinion, the consequences can vary.
Courts may exclude the testimony entirely, strike specific portions of a declaration, limit the scope of expert testimony, or impose other sanctions under Rule 37(c)(1), depending on the severity of the disclosure failure, the resulting prejudice, and whether the failure was substantially justified or harmless.
In deciding the appropriate remedy, courts often consider prejudice, the ability to cure the issue, and the impact on the case schedule.
When does supplementation become improper under Rule 26(e)?
Supplementation may become improper when it materially changes or expands previously disclosed opinions rather than correcting or completing disclosures. Courts evaluate whether the new information should have been included in the original report and whether the opposing party had a fair opportunity to respond.
Can an expert clarify their opinion after a deposition?
Yes, but only to explain existing opinions. Courts generally do not allow changes that materially alter prior testimony or introduce new reasoning that was not previously disclosed.
What rule allows courts to exclude improper expert declarations?
Rule 37(c)(1), which permits exclusion or other remedies unless the failure to disclose was substantially justified or harmless.
Are rebuttal declarations treated differently?
Yes. Rebuttal is typically limited to responding to opposing expert opinions and cannot be used to introduce new affirmative theories that should have been disclosed earlier.
Do courts exclude expert declarations that introduce new opinions?
Courts regularly consider exclusion when declarations introduce materially new opinions after disclosure deadlines, particularly where the opposing party lacks a fair opportunity to address them and the failure is not substantially justified or harmless.
The label attached to a declaration does not control the analysis. Courts focus on what the declaration actually does.
A declaration may be considered improper supplementation when it introduces opinions, reasoning, methodologies, or substantive analysis that were not fairly disclosed in the original expert report. Courts commonly explain that Rule 26(e) is not intended to provide an opportunity to cure deficiencies caused by incomplete preparation or to extend expert disclosure deadlines.
Common examples of improper supplementation include:
At the same time, not every addition will lead to exclusion. Courts evaluate these issues in context.
For example, an expert may provide additional explanation supporting an already disclosed opinion. Courts generally examine whether that explanation was fairly disclosed in substance in the original report and whether the opposing party had notice of the expert’s reasoning.
If the later declaration introduces materially different reasoning, relies on analysis that was not fairly disclosed in the original report, or advances a new theory, courts are more likely to treat it as an untimely new opinion rather than a permissible clarification.
Late changes to expert opinions can create real challenges in litigation.
They can limit the opposing party’s ability to prepare, disrupt discovery deadlines, and require additional motion practice. These concerns are central to how courts apply Rule 37(c)(1).
In Yeti by Molly, Ltd. v. Deckers Outdoor Corp., the Ninth Circuit affirmed exclusion of an expert damages report that was not properly disclosed during discovery. The court explained that exclusion under Rule 37(c)(1) operates as a self-executing sanction in the sense that a separate motion to compel is not required before exclusion may be imposed. At the same time, courts retain discretion to determine whether the failure was substantially justified or harmless.
Courts continue to apply that framework in later cases. In Goodman v. Staples The Office Superstore, LLC, the Ninth Circuit affirmed limitations on expert testimony where disclosure deficiencies prevented the opposing party from fairly addressing the expert’s opinions.
Exclusion is not automatic in every case. Courts retain discretion under Rule 37(c)(1) to determine whether a disclosure failure was substantially justified or harmless. Depending on the circumstances, courts may consider alternatives such as reopening discovery, allowing additional depositions, or limiting the scope of testimony.
Courts typically compare the expert’s original report, deposition testimony, and the later declaration side by side.
A declaration is more likely to be excluded when it:
When a declaration materially contradicts prior testimony, courts may also evaluate whether it improperly attempts to create factual disputes through inconsistent statements.
On the other hand, courts may allow additional explanation when it remains grounded in the original report. This can include elaborating on reasoning already disclosed or responding to criticisms raised by the opposing party, as long as the core opinions remain the same.
The key question is whether the opposing party had a fair opportunity to understand and respond to the expert’s opinions.
This issue often becomes critical at summary judgment, where expert declarations can determine whether a claim proceeds.
Because expert testimony is often necessary to establish elements such as causation or damages, exclusion at this stage can significantly affect case outcomes.
After a deposition, experts sometimes attempt to clarify or refine their opinions.
Courts often reject declarations that materially change prior testimony, especially when they appear to address weaknesses exposed during deposition. Limited clarification may be allowed if it remains consistent with the original report and testimony.
Parties may argue that a declaration is proper rebuttal, but courts focus on how the declaration functions.
Rebuttal is generally limited to addressing or contradicting the opposing expert’s opinions, rather than advancing new affirmative opinions that could reasonably have been disclosed earlier in the party’s case-in-chief.
Courts examine whether the declaration stays within the scope of responding to the opposing expert or instead expands beyond it.
Many state courts apply similar principles, although the specific rules and procedures vary.
For example, California’s expert exchange framework under Code of Civil Procedure provisions governing expert witness information exchange includes identification of expert witnesses, procedures for exchanging expert information, expert declarations in certain circumstances, deposition rights, and limitations on testimony beyond the scope of disclosed subject matter.
Across jurisdictions, courts focus on the same core concern: whether the opposing party had a fair opportunity to prepare for and respond to the expert’s opinions.
In practice, the safest approach is to treat the expert report as the foundation of the case.
To the extent reasonably foreseeable at the time of disclosure, the report should include the expert’s intended opinions, supporting analysis, methodology, and material assumptions. Courts may exclude testimony when reports fail to adequately explain the expert’s reasoning or methodology.
Depositions provide an opportunity to confirm the expert’s reasoning and address any ambiguity.
Focus on clarifying assumptions, testing the analysis, and ensuring that the expert’s opinions are fully developed on the record. Gaps left at this stage can be difficult to address later.
When preparing a declaration, stay grounded in the original disclosures.
A useful question is whether the declaration explains what is already in the record or introduces something new. If it introduces new substance, it may create a risk of exclusion under Rule 37(c)(1).
If the opposing party submits a questionable declaration, act promptly.
Compare the declaration to the expert report and deposition testimony. Identify any new opinions or inconsistencies and consider whether a motion to exclude is appropriate.
If a court finds that a declaration improperly supplements an expert opinion, the consequences can vary.
Courts may exclude the testimony entirely, strike specific portions of a declaration, limit the scope of expert testimony, or impose other sanctions under Rule 37(c)(1), depending on the severity of the disclosure failure, the resulting prejudice, and whether the failure was substantially justified or harmless.
In deciding the appropriate remedy, courts often consider prejudice, the ability to cure the issue, and the impact on the case schedule.
When does supplementation become improper under Rule 26(e)?
Supplementation may become improper when it materially changes or expands previously disclosed opinions rather than correcting or completing disclosures. Courts evaluate whether the new information should have been included in the original report and whether the opposing party had a fair opportunity to respond.
Can an expert clarify their opinion after a deposition?
Yes, but only to explain existing opinions. Courts generally do not allow changes that materially alter prior testimony or introduce new reasoning that was not previously disclosed.
What rule allows courts to exclude improper expert declarations?
Rule 37(c)(1), which permits exclusion or other remedies unless the failure to disclose was substantially justified or harmless.
Are rebuttal declarations treated differently?
Yes. Rebuttal is typically limited to responding to opposing expert opinions and cannot be used to introduce new affirmative theories that should have been disclosed earlier.
Do courts exclude expert declarations that introduce new opinions?
Courts regularly consider exclusion when declarations introduce materially new opinions after disclosure deadlines, particularly where the opposing party lacks a fair opportunity to address them and the failure is not substantially justified or harmless.
Federal Rules of Civil Procedure
Federal Rules of Civil Procedure