3. Confidence Interval for the Proportion, Large Samples
As long as n*p > 5 and n * q > 5, the sampling distribution of the proportion follows the normal distribution.
p +/- [ z * sp ]
where p = sample proportion
q = 1-p
sp = standard error of the proportion = sqrt(pq/n)
z = two-tailed z score for the desired level of confidence
Is this the correct formula for this problem?