In a cohort study, why might you want to start by selecting a defined population for study, rather than starting by selecting exposed and non-exposed individuals?

Texas’ gulf coastal plains are the western extension of the coastal plain extending from the atlantic ocean to beyond the rio grande. its characteristic rolling to hilly surface covered with a heavy growth of pine and hardwoods extends into east texas. in the increasingly arid west, however, its forests become secondary in nature, consisting largely of post oaks and, farther west, prairies and brushlands.

Yes thats true but what is the question