- When constructing the confidence interval a population mean, assuming the population standard deviation is known, we use the following formula: Critical Value Standard Error Sample Mean 0. i_ Za/ 2 Margin of Error The table below contains information about two confidence intervals. Use the given information to determine the missing values: Round answers accurate to 2 decimal places. The table below contains information about two confidence intervals. Use the given information to determine the missing values: Round answers accurate to 2 decimal places. f'd Con I ence 89.90% Qty" Level Critical Value 2.17 (2m)