The Economics of Common Impact in Antitrust Class Certification

36 Pages Posted: 10 May 2009 Last revised: 8 Sep 2011

See all articles by Paul A. Johnson

Paul A. Johnson

Bates White, LLC; Competition Bureau of Canada

Date Written: August 2011


With recent changes in case law requiring district courts to make deeper inquiries at the class certification stage, a systematic treatment of the economics of class certification takes on great importance. This article offers an economic interpretation of the legal concept of common impact as it is used in antitrust class certification. It interprets the predominance requirement central to Rule 23(b)(3) to mean that all economically significant factors are common factors. These economic factors may or may not be related to the conduct alleged by plaintiffs; thus, common impact should be investigated by examining the defendant conduct that plaintiffs allege violated antitrust law (“conduct factors”) as well as to determinants of price unrelated to the alleged conduct (“non-conduct factors”). With this economic interpretation established, this article proceeds to examine empirical testing of common impact and describes three types of tests that are likely to be applicable in a wide variety of antitrust class certification matters. Each type of test focuses on differences in prices paid by putative class members.

Keywords: class certification, common impact

JEL Classification: K21

Suggested Citation

Johnson, Paul A. and Johnson, Paul A., The Economics of Common Impact in Antitrust Class Certification (August 2011). Antitrust Law Journal, Vol. 77, No. 2, 2011, Available at SSRN:

Paul A. Johnson (Contact Author)

Competition Bureau of Canada ( email )

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Bates White, LLC ( email )

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