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Calculating the Overall Cpk

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  • #36059

    Jude
    Participant

    If a certain process has several parameters (with different specifications) being measured by Cpk, how do we calculate the overall Cpk? Is getting the weighted average an acceptable approach?  
     
     

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    #102990

    Bob J
    Participant

    Jude,
    No, a weighted average would not be mathematically valid…. 
    Right or wrong we consider the overall Cpk of a process to be the lowest Cpk value of a critical parameter…  The logic we use is that the process is only as good as the weakest critical parameter. 
    Hope this helps…
    Best Regards,
    Bob J

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    #102993

    Reigle Stewart
    Participant

    Jude: Convert each Cpk to a standard normal deviate;
    i.e., Z=Cpk*3. Next, you must convert Z to equivalent
    yield (in Excel use normsdist(Z)). After this, you need to
    multiply all of the yield values to get the rolled throughput
    yield … Y.rt = Y1 * Y2 * … * Yn, where n is the number of
    yield values (i.e., the number of Cpks you converted).
    Then normalize the rolled yield by computting Y.norm=
    Y.rt^(1/n). Finally, convert Y.norm to a Z value (using
    Excel) and then divide by 3 to get the final answer …
    Cpk.norm = Z.norm / 3. The resulting normalized Cpk is
    “kind of like an average” but is statistically valid. Kind
    Regards, Reigle.

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