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):

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

Assumptions:
– 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
– Data are representative for the process
– Process is stable (no special causes)
– Data are normally distributed
– The underlying s is calculated from 25-50 samples within a short time frame representing short term variation (i.e. days).

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