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statshelp
04-09-2008, 12:02 PM
Question: How large a sample do you need to estimate a mean with 95% confidence if the margin of error is 7% of the population standard deviation?

My work:
95% CL = 1.96
E= .07
stand. dev. = unknown

(1.96^2)*.25/(.07^2) = 196

Not sure if that is correct, please help

royhaas
04-09-2008, 12:34 PM
Why do you have a ".25" in your formula?

statshelp
04-09-2008, 12:47 PM
Because the standard deviation is Unknown.

royhaas
04-09-2008, 01:02 PM
You are calculating a sample size for a mean, with E = 0.07 \sigma. You are not calculating a sample size for a proportion. Please refer to your textbook.

statshelp
04-09-2008, 04:00 PM
Oh I see! So to find the sample size for the mean....

(1.96)/(.07^2)=28^2=784/4=196

But I get the same answer?