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Cpk & Ppk Target

Discussion in 'Capability - Process, Machine, Gage …' started by Pongsakorn, Feb 19, 2016.

  1. Pongsakorn

    Pongsakorn Member

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    What should be the proper Cpk and Ppk Target.
    QS9000 and VDA 6.3 requirement is totally different.

    QS9000 recommend Ppk >1.33 and Cpk >1.67 whereas
    VDA 6.3 require Ppk >/=1.67 and Cpk >/=1.33.

    In my opinion, I tend to agree with QS9000 since the Cpk usually
    greater than Ppk. I do not understand how VDA 6.3 determined such target.

    This is from QS9000
    upload_2016-2-19_18-30-31.png

    This is from VDA 6.3
    upload_2016-2-19_18-47-32.png
     

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  2. ncwalker

    ncwalker Well-Known Member

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    I think VDA 6.3 has "short term" and "long term" swapped in their text. I agree with QS.
     
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  3. Bev D

    Bev D Moderator Staff Member

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    The VDA doesn't have short term and long term swapped. they are using different formulas and data collection / sampling structures than QS is.
    both are correct given their definitions. Although some would argue (and theoretically correctly) that QS is wrong because you cannot have a chronically unstable yet predictable process...

    And of course the targets are atmospherically derived based on an erroneous assumption that the Normal distribution exists for everything, there actually IS data beyond +/-3 SD and that one can actually use a continuous data distribution to predict a categorical result AND that the specification limits actually are predictive of failure.

    ...so both are wrong since all capability indices are useless abominations on the face of the earth. but hey, that's just my opinion based on 35 years of continually improving quality without EVER calculating one of the darn things.
     
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  4. Bev D

    Bev D Moderator Staff Member

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    I have posted a brief article on these definitions in the resources section...
     
  5. Bob Doering

    Bob Doering Member

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    Also, QS9000 is obsolete, but when utilizing these indices you MUST understand their statistical limitations. These limitations are nicely laid out in AIAG PPAP 4th edition section 2.2.11.5 Note: The above mentioned acceptance criteria (2.2.11.3) assume normality and a two-sided specification (target in the center). So, first you need to find out if your distribution and specification apply before even bothering. Then, recognize that even if they do, their value is marginal as Bev D. indicated.
     

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