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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?

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    $\begingroup$ Related Question $\endgroup$ – Rory M Feb 15 '13 at 23:41
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    $\begingroup$ You might consider posting this on Cross Validated, but as shigeta mentioned, you'd need to provide more details. $\endgroup$ – blep Feb 16 '13 at 9:15
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    $\begingroup$ Maybe this isn't the answer you're looking for... nobody who has ever done a study like this has wanted to have fewer subjects than they could get. I'd say the answer to your question could just be 'funding' $\endgroup$ – shigeta Feb 18 '13 at 2:21
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    $\begingroup$ About experimental design and is a purely statistical question. $\endgroup$ – fileunderwater Oct 22 '13 at 11:54
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    $\begingroup$ I'm voting to close this question as off-topic because this question is about statistics, not biology. $\endgroup$ – Remi.b Jun 29 '17 at 15:37
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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.

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    $\begingroup$ Welcome to Biology.SE! This does not seem to answer the question. Please add some more details and references to your answer. $\endgroup$ – another 'Homo sapien' Jun 29 '17 at 15:05

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