The Formulas of Ppk and Cpk in VDA 4.1

J

jinzhoujun

#1
hi to all!
I know the Formulas of Ppk and Cpk in VDA 4.1 are different from the Formulas of Ppk and Cpk in AIAG's SPC manual,but I do not know exactly the Formulas of Ppk and Cpk in VDA 4.1 .who can help me ?thanks.
 
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J

jinzhoujun

#4
Thanks all of you two!
as far as I know ,in AIAG SPC manual Cpk is Process Capability Indies,Ppk is Process Performance Indies,Cpk>1.67,Ppk>1.33:but in VDA2 Cpk is long-term Process Capability ,Ppk is Preliminary Process Capability ,Cpk>1.33,Ppk>1.67.

Howard Atkins, I still need you help, I want know the Formulas of Ppk in VDA4.1. what is more,I want know the Formulas of σtot in page 137 of VDA 4.1 3rd edition.
 
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Howard Atkins

Forum Administrator
Staff member
Admin
#5
I think that some of the problems are the translations and the use of the various terms.
jinzhoujun said:
as far as I know ,in AIAG SPC manual Cpk is Process Capability Indies,Ppk is Process Performance Indies,Cpk>1.67,Ppk>1.33:but in VDA2 Cpk is long-term Process Capability ,Ppk is Preliminary Process Capability ,Cpk>1.33,Ppk>1.67.
There is a mistake in AIAG it should be ppk >1.67 and Cpk>1.33. Please check
I do not think that there is any real difference
 
J

jinzhoujun

#6
I think that some of the problems are the different definition of the terms.
in AIAG spc manual it should be ppk >1.33 and Cpk>1.67.I checked more times.
 

Howard Atkins

Forum Administrator
Staff member
Admin
#7
The requirement for 1.33 or 1.66 is in fact from the PPAP handbook and not from the SPC book as far as I understand
See page 6-7 of the 3rd edition PPAP handbook
 
J

jinzhoujun

#8
Thank you for your email to help me.
you are right,the requirment of Ppk and Cpkis from the PPAP handbook .
what I mean is that :according to SPC book ,it should be Cpk>Ppk;
according to VDA4.1,it should be Cpk<Ppk.
 
R

Rob Nix

#9
One is not necessarily higher that the other or required to be such. The customer decides what level of capability (1.33, 1.67, etc.) is required (although I usually see 1.67 for short term Ppk and 1.33 expected for long term Cpk).

Whether Ppk is called "process performance" or "preliminary process capability", all it really means is that all or most of the data is known - meaning that you can use standard deviation in the denominator. Cpk uses an estimate (e.g. some constant factor of a range value) to determine sigma, and is more often long term (as in the case of getting the data from current control charts).
 
G

gszekely

#10
As far as I know, your ideal state and target should be CPK=PPK. Am I wrong ?
So which one is for short and for long term variation ? Until now I have used CPK for short (1.67), and PPK for long(1.33), but it seems that I'm wrong, thus the info is froma transleated manual as well.
thanks
 
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