The Seventh Circuit held last week that a district court set too high a standard for class certification in an antitrust case.

"In essence," the panel ruled, it is important not to let a quest for perfect evidence become the enemy of good evidence." Messner v. Northshore University HealthSystem, No. 10-2514, slip op. 3 (7th Cir. Jan. 13, 2012).

The district court had refused to certify a class of suburban Chicago hospital patients who claimed that a merger hurt competition for their healthcare dollars and raised prices. The court thought that common issues didn't "predominate" under Rule 23(b)(3) unless the class could prove uniform increases in hospital prices for all services after the merger. But not all prices went up at the same rate.

The Seventh Circuit vacated the order denying certification. The panel concluded:

[T]he evidence shows that [plaintiffs' economics expert] Dranove can use common evidence and his difference-in-differences methodology to estimate the antitrust impact, if any, of Northshore’s merger on the members of that class. Together with the common questions and evidence on other liability issues, this was sufficient to show predominance under Rule 23(b)(3).

Id. at 45.

The defendants, by the way, had lost at trial before an administrative law judge on the Federal Trade Commission's claim that the merger created an unlawful monopoly under section 2 of the Sherman Act.