Process performance index: Difference between revisions

Revision as of 17:54, 22 July 2009

Revision as of 19:41, 30 March 2010

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==Interpretation==

Larger values of P<SUB>pk</SUB> may be interpreted to indicate that a process that is more capable of producing output within the specification limits, though this interpretation is controversial.{{cn}} Strictly speaking, from a statistical standpoint, P<SUB>pk</SUB> 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 <MATH>\hat{\mu}</MATH> and <MATH>\hat{\sigma}</MATH>.{{cn}} Furthermore, using this metric of past process performance to predict future performance is highly suspect.{{cn}}

Strictly speaking, from a statistical standpoint, P<SUB>pk</SUB> 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 <MATH>\hat{\mu}</MATH> and <MATH>\hat{\sigma}</MATH>. 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, P<SUB>pk</SUB> is a convenient metric to gauge how set-up is progressing (increasing P<SUB>pk</SUB> being interpreted as "the process capability is improving"). The risk is that P<SUB>pk</SUB> is taken to mean a process is ready for production before all the kinks have been worked out of it.