Question: In a cohort study, the advantage of starting by selecting a defined population for the study before any of its members become exposed, rather than starting by selecting exposed and non-exposed individuals, is ________________.
Answer: A number of exposures can be studied simultaneously.
Why isn't this the right answer? The study can be completed more rapidly.
If you start by selecting a population, then, only later, see which people were exposed and which were not, you might have to wait for a long time before enough people ...