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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 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 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 at 9:15
Thanks...actually that's the kind of information I was hoping to hear. The age is 5-10 years, geographical region is US, and family income is irrelevant (randomized). – Prabhu Feb 16 at 18:28
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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 at 2:21
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