1. Selection of Case and Control Subjects
As in all case–control studies, the definition of a ‘‘case’’ involves the establishment
of a clear, objective study end point and eligibility criteria for individuals
included in the study (1). Eligibility criteria restrict study participants to
those who are at risk for exposure (i.e., have the opportunity to be screened) and
are at risk for the study end point (i.e., death from the cancer under study). Case
and control subjects are identified from the source population by use of the same
eligibility criteria. In the next few sections, we discuss several issues that are of
concern in screening case–control studies, including potential source populations
for screening studies, special concerns in defining a case, and the selection of
control subjects.
A. Source Population