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Cpk vs Ppk

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  • #45105

    Capable
    Participant

    Does anyone know where there is an official definition of Cpk and Ppk.  I have looked at the PPAP manual, hundreds of forums and websites and everyone states something different.  No one can decide which is long term and which is short term or which should be 1.33 or 1.67. 
    The PPAP manual states
    Cpk – Short term 1.33
    Ppk – Long term 1.67
    But shouldn’t you be targeting the 1.67 for short term with less variables and 1.33 over the entire process.
    This is so anoying that there is no clear explaiantion, even though it appears each place I have looked believes they are correct.

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    #146328

    Mikel
    Member

    The offical definitions are in the process capability section of the SPC manual out of AIAG – it is what Minitab and Six Sigma’s definitions are based on. I believe it is on page 81 (don’t have it in front of me).
    Cpk is based on R-bar/d2 which is short term
    Ppk is based on s calcuated from the entire set of data which is long term (assuming the entire set of data has reasonably capture most sources of variation).

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    #146333

    Capable
    Participant

    Thanks for the information Stan.  That is what I have thought but I have been having conversations with a customer and they define Cpk and Ppk as the oppsoite.  I started looking on line and it appears that by what I have seen that many companies, websites and people are confused based on their defintion.
    My only other question is on 1.33 / 1.67. 
    Is it correct to be targeting a 1.33 cpk and a 1.67 Ppk?
    Should I not be targeting a the 1.33 Ppk for long term since I am taking into account all proces variable across sub groups.
     

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    #146336

    Mikel
    Member

    You are correct.

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    #146475

    Schmidt
    Participant

    If you have a customer using the AIAG Standard you are right…..
    If you have a customer using VDA or ANFIA then :
    Ppk is Critical preliminary process capability (should be > 1.66)
    Cpk is Critical process capability  (should be > 1.33)
    So what I learned for calculating process capability .. first ask your customer how he calculates, then do it the same way…

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