Confidence Interval for p
For large n, a confidence interval for p is given by
Determining The Sample Size
For error margin for the estimation of p, we use the expression . The required sample size is obtained by equating , where d is the specified error margin.
If the value of p is known to be roughly in the neighborhood of a value p*, then n can be determined from
Without prior knowledge of p, pq can be replaced by its maximum possible value 1/4 and n determined from the relation