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I am doing a sample size calculation for a survey and am using the formula found at the bottom of this page: http://www.raosoft.com/samplesize.html

In the formula, r is 'the fraction of responses that you are interested'. What is this exactly? I thought it would be the percentage of the population I want to look at, but this is essentially the sample size I need to examine, which is the outcome answer, so that can't be right.

How do you get a value for r?

Thanks!

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    $\begingroup$ Looks like it's referring to the response distribution value that you can enter at the top - i.e. the percentage of subjects you expect to be affected. $\endgroup$
    – naught101
    May 10 '12 at 0:30
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So basically 50%, well I will see if I can use this in the formula and get some sensible results.

Thanks for your response, I appreciate it!

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    $\begingroup$ To make this response complete and sustainable, would you perhaps share some more information with us? In particular, why 50% is the default choice for fraction of responses? $\endgroup$
    – chl
    May 10 '12 at 12:52

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