The Process Performance Indices P_{p} and P_{pk} are often referred to as 'long term' process capability measures. This is because the standard deviation used in their calculation is calculated using the 'Root Mean Square Error' (RMSE) method which is affected (inflated) by shifts in the process mean.
The Process Capability indices C_{p} and C_{pk} are referred to as 'short term' process capability measures because the standard deviation used in their calculation is calculated using the Range Method which is not affected by changes to the process mean.
There is much confusion and debate about the meaning of short and long term capability, so use these terms with care and be sure that everybody has the same understanding.
