Process Capability Indices(Cp and Cpk)

A standardised measurement on the short-term process performance of a process is Cp, nornally  equated to six standard deviations of the dispersion ;  Cp = (USL-LSL) / 6
The long term process performance, Cpk, has a similar ratio to that of Cp except that this ratio considers the shift of the mean relative to the target value ; Cpk = min{(USL-T) / 3 , (T-LSL) / 3