How large of a sample size is required?

Question

You want to obtain a sample to estimate a population proportion. Based on previous evidence, you believe the population proportion is approximately p = 0.38. You would like to be 95% confident that your esimate is within 2% of the true population proportion. How large of a sample size is required?

Answer 1

Hi Annie,

Again, we use the sample size equation:

n=(z*/m)²*P*q

z*=z-critical value

m=margin of error

p=probability estimate

q=1-p

Here:

z*=1.96--this is worth memorizing since 95% is a frequently used confidence level

m=0.02 (2% as decimal)

p=0.38

q=0.62

n= (1.96/0.02)²*0.38*0.62

n= 2262.70

Always round minimum sample sizes to the next whole number:

n= 2263

I hope this helps.