1 Answers
In process improvement efforts, the process performance index is an estimate of the process capability of a process during its initial set-up, before it has been brought into a state of statistical control.
Formally, if the upper and lower specifications of the process are USL and LSL, the estimated mean of the process is μ ^ {\displaystyle {\hat {\mu }}} , and the estimated variability of the process is σ ^ {\displaystyle {\hat {\sigma }}} , then the process performance index is defined as:
σ ^ {\displaystyle {\hat {\sigma }}} is estimated using the sample standard deviation. Ppk may be negative if the process mean falls outside the specification limits.
Some specifications may only be one sided. For specifications that only have a lower limit, P ^ p , l o w e r = μ ^ − L S L 3 σ ^ {\displaystyle {\hat {P}}_{p,lower}={{\hat {\mu }}-LSL \over 3{\hat {\sigma }}}} ; for those that only have an upper limit, P ^ p , u p p e r = U S L − μ ^ 3 σ ^ {\displaystyle {\hat {P}}_{p,upper}={USL-{\hat {\mu }} \over 3{\hat {\sigma }}}}.