qThe probability that a sample mean, x, lies between m-sx and m+ sx may be written: qHowever, it is more important to find out the confidence interval for the population mean m.. The previous relation can be written as: qBut we don’t know the true value of s (the standard deviation of the population). If the sample size is large enough (N³30), we can use s (the standard deviation estimated by samples) to approximate s: