Posted by **Johniece McCoy** on Friday, March 16, 2012 at 1:04pm.

If you want to be 95% confident of estimating the population proportion to within a sampling error of and there is historical evidence that the population proportion is approximately 0.40, what sample size is needed?

- statistics -
**MathGuru**, Friday, March 16, 2012 at 6:08pm
Formula to find sample size:

n = [(z-value)^2 * p * q]/E^2

... where n = sample size, z-value is 1.96, p = .4, q = 1 - p, ^2 means squared, * means to multiply, and E = ? (missing in problem).

Plug values into the formula and calculate n.

I hope this will help get you started.

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