Process performance index

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.cite book | last = Montgomery | first = Douglas | title = Introduction to Statistical Quality Control | publisher = John Wiley & Sons | date = 2004 | location = New York, New York | pages = 348 – 349 | url = | isbn = 9780471656319 | oclc = 56729567

Formally, if the upper and lower specifications of the process are USL and LSL, the estimated mean of the process is hat{mu}, and the estimated variability of the process (expressed as a standard deviation) is hat{sigma}, then the process performance index is defined as::hat{P}_{pk} = min Bigg [ {USL - hat{mu} over 3 imes hat{sigma, { hat{mu} - LSL over 3 imes hat{sigma Bigg]

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

Some specifications may only be one sided (for example, strength). For specifications that only have a lower limit, hat{P}_{p,lower} = {hat{mu} - LSL over 3 imes hat{sigma; for those that only have an upper limit, hat{P}_{p,upper} = {USL - hat{mu} over 3 imes hat{sigma.

Practitioners may also encounter hat{P}_{p} = frac{USL - LSL} {6 imes hat{sigma, a metric that does not account for process performance that is not exactly centered between the specification limits, and therefore is interpreted as what the process would be capable of achieving if it could be centered and stabilized.


Larger values of Ppk may be interpreted to indicate that a process that is more capable of producing output within the specification limits, though this interpretation is controversial.

Strictly speaking, from a statistical standpoint, Ppk is meaningless if the process under study is not in control because one cannot reliably estimate the process underlying probability distribution, let alone parameters like hat{mu} and hat{sigma}. Furthermore, using this metric of past process performance to predict future performance is highly suspect.

From a management standpoint, when an organization is under pressure to set up a new process quickly and economically, Ppk is a convenient metric to gauge how set-up is progressing (increasing Ppk being interpreted as "the process capability is improving"). The risk is that Ppk is taken to mean a process is ready for production before all the kinks have been worked out of it.

Once a process is put into a state of statistical control, process capability is described using process capability indices, which are formulaically identical to Ppk (and Pp). The indices are named differently to call attention to whether the process under study is believed to be in control or not.

ee also

*Process capability index


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