How large a sample needed to estimate mean w/ 95% conf.?

[statshelp]

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]

Re: How large a sample....?

Why do you have a ".25" in your formula?

 

[statshelp]

Re: How large a sample....?

Because the standard deviation is Unknown.

 

[royhaas]

Re: How large a sample....?

You are calculating a sample size for a mean, with \(\displaystyle E = 0.07 \sigma\). You are not calculating a sample size for a proportion. Please refer to your textbook.

 

[statshelp]

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?