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
deviation calculated using
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.