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.

Calculation:

Ppk = Min ( (USL – X-bar)/3sLT, (X-bar – LSL)/3sLT )

Assumptions:

  • Data are representative of the process
  • The process is stable (no special causes)
  • Data are normally distributed

Attention:

  • In contrast to the Ppk Index, the Cpk index is used to describe the capability of the process considering long term variation.

Read also: Cp, Pp, Ppk, Process Capability