Process performance index

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 ${\displaystyle {\hat {\mu }}}$, and the estimated variability of the process (expressed as a standard deviation) is ${\displaystyle {\hat {\sigma }}}$, then the process performance index is defined as:

${\displaystyle {\hat {\sigma }}}$ is estimated using the sample standard deviation. P_pk may be negative if the process mean falls outside the specification limits (because the process is producing a large proportion of defective output).

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