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Expert Witness Insights by Area of Expertise
5 min read
Areas of Expertise :
Class Action, Civil Litigation
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Mass torts and class actions both involve claims affecting multiple people, but they differ in claim structure, case management, certification requirements, and the role of individualized proof. This article explains how mass torts, multidistrict litigation, and Rule 23 class actions differ, and how expert witnesses may address common or claimant-specific issues involving causation, exposure, damages, statistics, and other technical evidence.
Mass torts and class actions can both involve claims by many people, but they are not interchangeable. In federal practice, mass-tort cases may be centralized for coordinated pretrial proceedings while remaining separate actions. A class action, by contrast, allows one or more representative parties to seek to litigate on behalf of a class, subject to certification under Rule 23.
This guide explains how mass torts and class actions differ, how multidistrict litigation fits into that framework, and how expert analysis may vary when proof is common or individualized. The discussion focuses on federal procedure, and state law can differ.
“Mass tort” generally describes litigation involving many claims arising from an identifiable event, product, or widespread exposure. It is not a single federal procedural mechanism.
When civil actions pending in different federal districts share one or more common questions of fact, the Judicial Panel on Multidistrict Litigation may transfer them for coordinated or consolidated pretrial proceedings. The Panel must determine that transfer will serve the convenience of the parties and witnesses and promote the just and efficient conduct of the actions.
Centralization can reduce duplicative discovery and inconsistent pretrial rulings, but it does not convert separate actions into a class action. Federal Rule of Civil Procedure 16.1, effective December 1, 2025, addresses early MDL management, including the parties’ views on leadership counsel, consolidated pleadings, discovery, factual and legal issues, and likely pretrial motions.
A class action is a representative lawsuit in which one or more class representatives litigate on behalf of a defined group. In federal court, the proposed class must satisfy Rule 23(a), including numerosity, commonality, typicality, and adequacy, as well as the requirements of an applicable Rule 23(b) category.
For classes seeking certification under Rule 23(b)(3), common questions must predominate over individual questions, and a class action must be superior to other available methods. Class members do not necessarily receive equal payments because distributions may account for differences among claims.
Mass torts and class actions differ primarily in how claims are structured, managed, and proven.
The number of claimants alone does not determine whether claims are filed individually, whether related federal actions are centralized in an MDL, or whether Rule 23 class treatment is available. These procedural mechanisms can overlap, and the applicable rules, claims, case posture, and mix of common and individualized issues also matter.
Litigation
Litigation Support
Causation Analysis
Damages Calculations
Evidence Analysis
Mass torts and class actions can both involve claims by many people, but they are not interchangeable. In federal practice, mass-tort cases may be centralized for coordinated pretrial proceedings while remaining separate actions. A class action, by contrast, allows one or more representative parties to seek to litigate on behalf of a class, subject to certification under Rule 23.
This guide explains how mass torts and class actions differ, how multidistrict litigation fits into that framework, and how expert analysis may vary when proof is common or individualized. The discussion focuses on federal procedure, and state law can differ.
“Mass tort” generally describes litigation involving many claims arising from an identifiable event, product, or widespread exposure. It is not a single federal procedural mechanism.
When civil actions pending in different federal districts share one or more common questions of fact, the Judicial Panel on Multidistrict Litigation may transfer them for coordinated or consolidated pretrial proceedings. The Panel must determine that transfer will serve the convenience of the parties and witnesses and promote the just and efficient conduct of the actions.
Centralization can reduce duplicative discovery and inconsistent pretrial rulings, but it does not convert separate actions into a class action. Federal Rule of Civil Procedure 16.1, effective December 1, 2025, addresses early MDL management, including the parties’ views on leadership counsel, consolidated pleadings, discovery, factual and legal issues, and likely pretrial motions.
A class action is a representative lawsuit in which one or more class representatives litigate on behalf of a defined group. In federal court, the proposed class must satisfy Rule 23(a), including numerosity, commonality, typicality, and adequacy, as well as the requirements of an applicable Rule 23(b) category.
For classes seeking certification under Rule 23(b)(3), common questions must predominate over individual questions, and a class action must be superior to other available methods. Class members do not necessarily receive equal payments because distributions may account for differences among claims.
Mass torts and class actions differ primarily in how claims are structured, managed, and proven.
The number of claimants alone does not determine whether claims are filed individually, whether related federal actions are centralized in an MDL, or whether Rule 23 class treatment is available. These procedural mechanisms can overlap, and the applicable rules, claims, case posture, and mix of common and individualized issues also matter.
Experts do not decide whether cases are centralized in an MDL or whether a proposed class is certified. Those procedural decisions are made by the appropriate court or panel. Expert analysis may instead address common proof, causation, damages, and differences among claimants.
Depending on the allegations:
Expert analysis may address issues that a party contends can be evaluated through common evidence or methodology. Depending on the dispute, economists, statisticians, survey experts, or other qualified specialists may analyze datasets, sampling, survey evidence, common impact, or classwide damages methodologies.
When expert analysis is offered on a Rule 23 issue, the methodology should fit the issue being addressed and comply with the evidentiary and procedural standards applicable to that proceeding. An expert should not assume that every class member experienced the same injury or loss unless the evidence supports that premise.
Under Federal Rule of Evidence 702, the party offering expert testimony must show the court that it is more likely than not that the rule's requirements are satisfied.
The testimony must help the trier of fact, rest on sufficient facts or data, and be based on reliable principles and methods. Any resulting opinion must also reflect a reliable application of those principles and methods to the facts of the case.
In both mass torts and class actions, an expert's qualifications should fit the disputed issue, and the expert should be able to explain the data, assumptions, methodology, and limitations underlying the opinion.
Is an MDL the Same as a Class Action?
No. An MDL coordinates or consolidates separate federal actions for pretrial proceedings under 28 U.S.C. § 1407. A federal action may be filed on behalf of a proposed class, but the case proceeds on behalf of a certified class only if the applicable Rule 23 requirements are satisfied.
Do Class Members Always Receive the Same Amount?
No. Class relief is not necessarily divided equally and can depend on the judgment, settlement terms, claims process, and differences among class members. For a proposed settlement that would bind the class, Rule 23(e)(2) requires the court to consider whether the proposal treats class members equitably relative to one another. The rule does not impose a categorical requirement that every class member receive the same payment.
Can Every Class Member Opt Out?
No. Rule 23(b)(3) classes provide an opportunity to request exclusion. Rule 23(b)(1) and Rule 23(b)(2) classes do not carry the same Rule 23 opt-out right, so the applicable class category matters.
What Experts Are Commonly Used in Mass Torts and Class Actions?
The disciplines depend on the claims and may include medicine, toxicology, epidemiology, engineering, statistics, economics, and damages analysis. The expert's role should address a specific technical or factual issue within the expert's qualifications.
Experts do not decide whether cases are centralized in an MDL or whether a proposed class is certified. Those procedural decisions are made by the appropriate court or panel. Expert analysis may instead address common proof, causation, damages, and differences among claimants.
Depending on the allegations:
Expert analysis may address issues that a party contends can be evaluated through common evidence or methodology. Depending on the dispute, economists, statisticians, survey experts, or other qualified specialists may analyze datasets, sampling, survey evidence, common impact, or classwide damages methodologies.
When expert analysis is offered on a Rule 23 issue, the methodology should fit the issue being addressed and comply with the evidentiary and procedural standards applicable to that proceeding. An expert should not assume that every class member experienced the same injury or loss unless the evidence supports that premise.
Under Federal Rule of Evidence 702, the party offering expert testimony must show the court that it is more likely than not that the rule's requirements are satisfied.
The testimony must help the trier of fact, rest on sufficient facts or data, and be based on reliable principles and methods. Any resulting opinion must also reflect a reliable application of those principles and methods to the facts of the case.
In both mass torts and class actions, an expert's qualifications should fit the disputed issue, and the expert should be able to explain the data, assumptions, methodology, and limitations underlying the opinion.
Is an MDL the Same as a Class Action?
No. An MDL coordinates or consolidates separate federal actions for pretrial proceedings under 28 U.S.C. § 1407. A federal action may be filed on behalf of a proposed class, but the case proceeds on behalf of a certified class only if the applicable Rule 23 requirements are satisfied.
Do Class Members Always Receive the Same Amount?
No. Class relief is not necessarily divided equally and can depend on the judgment, settlement terms, claims process, and differences among class members. For a proposed settlement that would bind the class, Rule 23(e)(2) requires the court to consider whether the proposal treats class members equitably relative to one another. The rule does not impose a categorical requirement that every class member receive the same payment.
Can Every Class Member Opt Out?
No. Rule 23(b)(3) classes provide an opportunity to request exclusion. Rule 23(b)(1) and Rule 23(b)(2) classes do not carry the same Rule 23 opt-out right, so the applicable class category matters.
What Experts Are Commonly Used in Mass Torts and Class Actions?
The disciplines depend on the claims and may include medicine, toxicology, epidemiology, engineering, statistics, economics, and damages analysis. The expert's role should address a specific technical or factual issue within the expert's qualifications.
Litigation
Litigation Support
Causation Analysis
Damages Calculations
Evidence Analysis
Expert Witness Insights by Area of Expertise
Expert Witness Insights by Area of Expertise