Class actions are not like other lawsuits. In a class action, an individual asks the court for permission to represent a group of people who have all been harmed in a similar way. As a result, the single most important debate in a class action is over certification—whether the lawsuit may proceed on behalf of a group of “similarly situated” people. And the single most important debate in certification is the debate over common issues: are there enough common issues to justify certifying a class for litigation, and possibly trial? ...