Using Excel to Find Critical Values Excel's NORM.S.INV function (inverse of the standard normal distribution) is used to find a critical value a/2. In using this function, it is very helpful to refer to Figure 7-1 or Figure 7-2 to see the relationships among the confidence level, the val- ues of a and a/2, and the critical value. Use the following procedure. 1. Given a confidence level, such as 95%, first express the confidence level as a decimal, such as 0.95. 2. Find the value of "1- confidence level" expressed as a decimal, and denote that value by a. For a confidence level of 95%, a = 1 = 0.95 0.05. 3. Find the value of a/2, which is a decimal number, such as 0.025. 4. Subtract the result from Step 3 from 1 to get a value of I if a 0.05, the value of 1 a/2 is 1 0.025 = 0.975. a/2. For example, 5. The Excel function NORM.S.INV is now used to find the z score correspond- ing to the result from Step 4. First, click the Insert Function f, button in the Formula Bar. 6. The Insert Function dialog box now appear