Step 2 The lower bound of the confidence interval is calculated using the expression x Za 2 V rounding the final value to two decimal places 7 33 O x a n x za 2 O n The upper bound is found in a similar fashion but now we will add the margin of error instead of subtracting it Again round the final value to two decimal places 7 50 33 50 Substitute the values x 33 Za 2 1 645 o 7 and n 50 into this formula and simplify Submit Skip you cannot come back Thus a 90 confidence interval for the population mean in a sample of 50 items with a sample mean of 33 and a population standard deviation of 7 is to