How large of sample is required estimate the population prop

How large of sample is required estimate the population proportion p, to 90% confidence, with a margin of error of 2%? Use the worst case scenario of p = 0.5.

Solution

p = 0.5

q = 0.5

sd/sqrt(n) = 0.02

sqrt(p*q)/0.02 = sqrt(n)

0.5*0.5/0.02 = n

n = 0.25 * 50 = 12.5