The 23(a) criteria are referred to as numerosity, commonality, typicality, and adequacy. Numerosity refers to the number of people in the class. To be certified, the class has to have enough members that simply adding each of them as a named party to the lawsuit would be impractical. There is no bright-line rule to determine numerosity, but classes with hundreds of members are generally deemed to be sufficiently numerous. To satisfy commonality, there must be a common question of law and fact such that "determination of its truth or falsity will resolve an issue that is central to the validity of each one of the claims in one stroke". The typicality requirement ensures that the claims or defenses of the named plaintiff are typical of those of everyone else in the class. Finally, adequacy requirement states that the named plaintiff must fairly and adequately represent the interests of the absent class members.