Date:         Tue, 29 Aug 2000 16:32:08 -0700
Reply-To:     Cassell.David@EPAMAIL.EPA.GOV
Sender:       "SAS(r) Discussion" <SAS-L@LISTSERV.UGA.EDU>
From:         "David L. Cassell" <Cassell.David@EPAMAIL.EPA.GOV>
Subject:      Re: What is the Difference between CPK & PPK?
Content-type: text/plain; charset=us-ascii

Tom Pruitt wrote [with regard to Cpk and Ppk]:
> What is the difference between these 2 indexes?

The difference is more-or-less the difference between the estimated
variance and the expected variance.  They will produce similar numbers
[most of the time] but they mean different things.

Cpk [and the related capability indices like Cp and Cr] should be used
when you want to work with the *estimated* sigma, and you want to
measure the capability of your system to meet the needs of the
customer or consumer.

Ppk [and the related indices like pp and Pr] should be used when you want
to work with the *calculated* sigma (the sigma based on the individual
observations) and you want to measure the actual performance of your
system to meet the stated [customer or consumer] needs.  So use Ppk when
you want to measure the system's *real* performance.

David
--
David Cassell, OAO Corp.                           Cassell.David@epa.gov
Senior computing specialist
mathematical statistician