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Z Value and Process Capability

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Viewing 8 posts - 1 through 8 (of 8 total)
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  • #53532

    singh
    Member

    Hey Guys

    Can anybody tell me the difference between the Z value and Cp, Cpk?

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    #190507

    Mikel
    Member

    Cpk = 3 * z

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    #190515

    Rivas
    Participant

    I think that Z=3*cpk.
    Z is sigma level. If the best cpk=2, sigma level = 6

    0
    #190516

    Rivas
    Participant

    Cp= (LSU – LSL) / 6stdv
    _
    Cpku= (LSU – X)/3stdv
    _
    Cpkl= (X-LSL)/3stdv

    Cpk = the lesser of two

    See me Linked In!

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    #190590

    MBBinWI
    Participant

    Marcelo caught you on that one Stan.
    However, the answer was incomplete. Z and Cp are not related mathematically (nor is Cpk related to Cp mathematically). Only in the instance where your distribution is perfectly centered between the LSL and USL will Cp = Cpk = Z/3 and this is not a mathematical relation, rather a definitional relation (yes, Stan, I am well aware that you understand this, I’m pontificating for the masses).

    1
    #190593

    Furgeson
    Member

    MBBinWI,

    We have been looking for you. We need to know if you are Stan’s mentor or his legal guardian.

    Please advise,

    t.

    0
    #190606

    MBBinWI
    Participant

    I could be a real smart alec and accept mentorship, but that would probably get me canned.
    And you certainly aren’t going to pin guardianship on me.

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    #248471

    subhayu
    Participant

    @MBBinWI

    I have a follow up question how is it that Z/3 = Cp/Cpk if the process is centered? I understand if the process is centered Cp=Cpk and both values of Cpk will be the same. What I dont get is how will that be equal to Z/3 assuming the Z statistic is Z=[X-(mu)]/(sigma)

    Will appreciate a response since I am due to take the CSSBB exam next month

     

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