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Any One Could Tell Me When I Should Use Cpk or Ppk? Thks!
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Posted by: Jim Parnella
Posted on: Thursday, 7th June 2001, 12:00 AM.
Preston,
To add to the information already posted:
Cpk is calculated using RBar/d2 or SBar/c4 for Sigma in the denominator of you equation. This calculation for Sigma REQUIRES the process to be in a state of statistical control. If not in control, your calculation of Sigma (and hence Cpk) is useless - it is only valid when in-control. Cpk is called the "process CAPABILITY index". Think of it this way ... Cpk tells you what the process is CAPABLE of doing in future, assuming it remains in a state of statistical control.
Ppk uses the sample standard deviation in the denominator. It does not require statistical control to be valid. Ppk is called the "process PERFORMANCE index". Think of it this way ... Ppk tells you how the process has performed in the past. You cannot use it predict the future, like with Cpk, because the process is not in a state of control.
Think of it a different way: You can use Cpk as a measure of PAST performance and FUTURE capability (assuming statistical control is maintained). Ppk however can only be used as a measure of PAST performance.
Also, like stated previously, the values for Cpk and Ppk will converge to almost the same value when the process is in statistical control. that is because Sigma and the sample standard deviation will be identical (at least as can be distinguished by an F-test). When out of control, the values will be distinctly different, perhaps by a very wide margin.
I hope this helps you decide which to use.
Jim
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