The Process Performance Indices Pp and Ppk are calculated in the same way as Cp and Cpk, except that the standard deviation is calculated using the RMSE method, not the Range Method
Pp and Ppk are measures of the long term performance of a process, whereas Cp and Cpk measure the short term performance.
The reason for the difference is that the standard deviation calculated using the RMSE method includes the variation caused by shifts in the process mean. The standard deviation calculated using the Range method is not affected by mean shifts.
