In population-based cohort studies, a sample, or
even the entirety, of a defined population is selected
for longitudinal assessment of exposure-outcome relations.
Because of their typically high cost and logistic
complexities, population-based cohort studies generally
evaluate multiple hypotheses, some defined a priori
as well as some suggested in the course of the study
either based on interim analyses or on advances in the
field, particularly when data are repeatedly collected
on cohort members. Perhaps the most often cited justification
for conducting a population-based study is
its external validity—that is, the applicability of its
results to a defined population