The first section on the sheet covers the calculations for the capability index Cp. Remembering back to the section on capability, you will recall that this value tells us how well the natural process spread could be contained within the customer's specification limits. In order to proceed, therefore, we first need to calculate the total process spread.
For a process with a normal distribution, all of the outputs should lie within 3 sigma units above and below the mean, which means that the natural process spread should be 6 sigma.
Note: In the previous exercise we calculated a value of 3 sigma for the subgroup means. When calculating capability indices, we are concerned with the individual values, which will have a wider spread than the subgroup means. Consequently we cannot simply use the 3 sigma value from the previous exercise, we have to calculate a new sigma value for the individual values.