CEVAP:

There are two important situations in the calculation of confidence interval for a population mean. The first case is that we use sample data to estimate the unknown value of population mean µ with the value of sample mean found from a random sample of size n, and also the population standard deviation ? is known beforehand. The second case is that we use sample data to estimate the unknown value of population µ with the value of sample mean found from a random sample of size n, and also the population standard deviation ? is not known. In the second case, instead of the population standard deviation ?, the sample standard deviation s is used as the substitute.