A confidence interval for a population mean is an estimate of the population mean together with an indication of reliability. There are different formulas for a confidence interval based on the sample...A confidence interval for a population mean is an estimate of the population mean together with an indication of reliability. There are different formulas for a confidence interval based on the sample size and whether or not the population standard deviation is known. The confidence intervals are constructed entirely from the sample data (or sample data and the population standard deviation, when it is known).