Why in case-control studies, cases and non-case are taken from "Two-different Population?"
Why don't they come from a single population?
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Let's take the definition of population in statistics:
population is a complete set of items that share at least one property in common that is the subject of a statistical analysis
this is a quote from simple definition on Wikipedia. So, you define your population of interest precisely and then sample from this population. Comparing cases to controls means that you want to compare two samples taken from two populations, e.g. population of marijuana smokers, to population of non-marijuana smokers. This is a simplified answer, however as it is a
self-study question, I hope it'll lead you a little bit further.