A Cohort is generally defined as a group of people within a population who are assumed to have identical exposures during a specified exposure period. The use of Cohorts is a necessary simplifying assumption for modeling exposures of a large population. For the exposure assessment, the population was divided into a set of Cohorts such that
1) each person is assigned to one and only one Cohort, and
2) all the Cohorts combined encompass the entire population.