Question

If you do not know the population standard deviation what should you do in picking a sample size?

Answer 1

If we don't know the population standard deviation, we will estimate the population standard deviation by sample standard deviation. For the further study of sample, it will be easy if the distribution of sample is normal distribution. so, to approximate to the normal distribution,

Central Limit Theorem states that we can approximate the sample distribution to normal distribution if the independent sample size is greater than 30. so we should pick a sample size of sample greater than 30.