Based on information in my intro to statistics textbook (I'm not a
professional statistician), there are ways of determining sample size
for a given confidence limit like you want to do. Exact formulas
depend on what type of data you have ("continuous" data or binomial
data).

Assuming my stat textbook is representative of all intro to stat texts,
you should be able to look this up in a textbook.


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MrShorty
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