The Ppk-Index compares the distance from the process center to the nearest Specification Limit (how the process should be) to the process spread (how it is). The long-term standard deviation sLT is calculated using long term data, usually 25-50 samples within a longer time frame representing all process variation – i.e. months.
Ppk = Min ( (USL – X-bar)/3sLT, (X-bar – LSL)/3sLT )
- Data are representative of the process
- The process is stable (no special causes)
- Data are normally distributed
- In contrast to the Ppk Index, the Cpk index is used to describe the capability of the process considering long term variation.