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

#### statshelp

##### New member
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

#### royhaas

##### Full Member
Re: How large a sample....?

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

#### statshelp

##### New member
Re: How large a sample....?

Because the standard deviation is Unknown.

#### royhaas

##### Full Member
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

##### New member
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?