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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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Related Question –  Rory M Feb 15 '13 at 23:41
    
this is a question of statistical experiment design. The statistical power of the experiment must include all the factors you are controlling for - age, geographical region, family income etc etc. So this really depends. not sure this can be answered here without a lot more information. –  shigeta Feb 16 '13 at 0:32
    
You might consider posting this on Cross Validated, but as shigeta mentioned, you'd need to provide more details. –  dd3 Feb 16 '13 at 9:15
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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' –  shigeta Feb 18 '13 at 2:21
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About experimental design and is a purely statistical question. –  fileunderwater Oct 22 '13 at 11:54
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