Ppk is a short term analysis (mostly done with 50 or 100 samples measured) Ppk is Process Potential Capability. To calculate the Cpk values, you need to estimate the standard deviation (s) from the range chart and the overall process average from the X chart. Process Capability & Performance (Pp, Ppk, Cp, Cpk) are indicators of the. Process Performance (Pp, Ppk) help you determine if your new process is capable. So the key takeaway is that Cpk is the potential of a process to meet a specification (short term) while Ppk is how the process actually did (long term). The only difference lies in the denominator for the Upper and Lower statistics: Cpk is calculated using the WITHIN standard deviation, while Ppk.