Confidence Interval Formulas: ********************************************************************************************** Using Standard Deviation of a Sample Conf Interval-Low Boundary = Mean - (z-value)(Standard Deviation) / (square root of sample size) Conf Interval-High Boundary = Mean + (z-value)(Standard Deviation) / (square root of sample size) Using Standard Deviation of the Underlying Population Conf Interval-Low Boundary = Mean - (z-value)(Standard Deviation) Conf Interval-High Boundary = Mean + (z-value)(Standard Deviation) Margin of Error - Population Standard Deviation Known (z-value)(Standard Deviation) Margin of Error - Population Standard Deviation Unknown, using the Sample’s Std Dev (z-value)(Standard Deviation) / (square root of the sample size) ********************************************************************************************** z-value Critical Values: 1.65 90% confidence 1.96 95% confidence 2.58 99% confidence ********************************************************************************************** |