P live within six miles of the Portsmouth Gaseous Diffusion Plant. They claim that they and their properties have been exposed to radioactive materials and non-radioactive hazardous wastes emitted from the Portsmouth plant. They have asked this Court to certify a class of all similarly-situated persons in order that they may present, on a classwide basis, claims for emotional distress, diminution in the value of their real property, medical monitoring for early cancer detection, and injunctive relief against further unlawful emissions of hazardous substances from the plant. The facts can be subdivided into two categories: those relating to class definition (that is, evidence that something occurred to distinguish the members of the class from the general public), and to class size.