Cpk value of 1.33 vs. a Ppk value of 1.67

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    I consider a Cpk value greater than 1.33 to be acceptable, however a new customer is requesting to comply with a Ppk value greater than 1.67
    Where does the 1.67 value come from?  and how it compares to the 1.33 Cpk value.


    Bob J

    A Ppk value of 1.67 gives you a Z of 5 for your long term capability…
    The 1.33 Cpk yields a short term Z of 4….
    Since a 1.33 Cpk value is the standard in many industries I assume that the customer preceives the 1.67 Ppk to be a tighter requirement.
    Hope this helps….
    Bob J



    The Cp metric reflects the potential capability of the process assuming that the histogram is positioned cantrally within the speciation limits (VOC).
    Cp values of 1.33 or above are considered accaptable whilst above 2 is excellent.
    The Cpk metric reflects the actual capability of the process by measuring the same ratio as the Cp, but only the nearest specification limit, since this is the limit which is most likely to be failed.



    Knowing that Ppk relates to long term capability which contains more variation than the short term (Cpk), most organizations that I’ve had experience with may require a 1.67 Cpk and an associated 1.33 Ppk. I suspect that your customer may be looking for a Cpk of 2.0 or higher, but you should confirm this with them.
    Also, the difference between a 1.33 Cpk and a 1.67 Ppk relates to a sigma shift of 1 (not the “magic” 1.5 typically used).
    Good luck.

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