Evan Tager is a Partner with Mayer Brown LLP in the firm’s Washington, DC office and is the WLF Legal Pulse’s Featured Expert Contributor on Judicial Gatekeeping of Expert Evidence. Jonathan S. Klein, an Associate, with the firm. Along with five other Mayer Brown attorneys, they co-authored a January 2020 WLF Monograph, Admissibility of Expert Testimony: Manageable Guidance for Judicial Gatekeeping.

Plaintiffs often must use expert testimony to support their motions for class certification, especially in attempting to show commonality under Federal Rule of Civil Procedure 23(a)(2) and predominance under Rule 23(b)(3) (the WLF Monograph noted above discussed this issue at length). A recent Ninth Circuit decision that affirms a district court’s exclusion of expert testimony, and consequently affirms the denial of class certification, underscores the unreliability of an expert opinion that purports to extrapolate from a miniscule sample to a nationwide class that comprises tens or hundreds of thousands of consumers.