Say I'm designing a retrospective study (where I'd collect data from various schools for the last 5 years) to see if consuming milk can contribute to childhood obesity. How do I decide on how many schools to visit? If I remember my stats correctly, in general, the goal is to achieve 95% confidence in our results, right? So how many schools would I have to collect data from so that I will have 95% confidence in the outcome of the study?
closed as off-topic by Remi.b, Bryan Krause♦, theforestecologist♦, David, another 'Homo sapien' Jun 30 '17 at 3:16
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Could be as much as possible – using multistage sampling. But it all depends on how much you are willing to spend, and your time constraint.