Posted by **ali** on Tuesday, July 26, 2011 at 2:16pm.

You want to estimate the grades of st2%atistics students who gets grades of C or better. How many students should you survey to be 95% confident that the sample percentage is off by no more than 2 percentage points.

- statistics -
**MathGuru**, Tuesday, July 26, 2011 at 7:27pm
Try this formula:

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

With your data:

n = [(1.96)^2 * .5 * .5]/.02^2

(Finish the calculation and round to the next highest whole number.)

Note: n = sample size needed; .5 for p and .5 for q are used if no value is stated in the problem. E = maximum error, which is .02 (2%) in the problem. Z-value is found using a z-table (for 95%, the value is 1.96).

I hope this will help.

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