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November 28th, 2015, 05:55 AM   #1
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sample size for confidence interval

Question: A state legistlator wishes to survey residents of her district to see what proportion of the electorate is aware of her position on using state funds to pay for abortions. What sample size is necessary if the 95% confidence interval for p is to have width at most 0.10 irrespective of p?

(p bar) + Z0.025 sigma/sqrt(n) - (p bar - Z0.025 sigma / sqrt(n)) <= 0.1
2(Z0.025 sigma/ sqrt(n) ) <= 0.1

but sigma is not known

Last edited by tttim; November 28th, 2015 at 06:27 AM.
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November 28th, 2015, 02:50 PM   #2
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Sigma is a function of p and n (binomial distribution). Fixing sigma leaves n dependent on p.
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confidence, interval, sample, size

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