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Any One Could Tell Me When I Should Use Cpk or Ppk? Thks!
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Posted by: Eoin
Posted on: Thursday, 7th June 2001, 12:00 AM.
Interesting discussion. I find that the similarity differences between PpK and Cpk can give a lot of insight into a process and how to approach improving it.
Cpk is calcualted using an estimate of the standard deviation caluculated using R-bar/d2.
PpK uses the usual form of the standard deviation ie the root of the varience or the square root of the sum of squares divided by n-1.
The R-bar/D2 estimation of the standard deviation has a smoothing effect and the Cpk statistic is less sensitive to points which are further away from the mean than is Ppk.
As the Ppk and Cpk converge the reliability of the process increases. This has implications for the effectiveness of you experimentation.
It could be argued that the use of Ppk and Cpk (with sufficient sample size) are far more valid estimates of long and short term capability of processes since the 1.5 sigma shift has a shaky statistical foundation.
I prefer to use Ppk when speaking to our internal customers when determining the performance of the process and when setting goals and metrics.
Hope this is useful.
Best wishes,
Eoin
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