iSixSigma

ScottS

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  • #83555

    ScottS
    Member

    Cpk came from the Japan at the latest in the 70’s.Β  What does the “k” mean?Β  Obvioulsy nothing in English, but it stands for “katayori” which is shift or offset in Japaneese.
    Β 
    See previous thread on topic of origins of Cpk.

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    #83534

    ScottS
    Member

    Attached is explanation of subjet from Minitab.Β  On a related note, Minitab has made a macro on my request to not calculate Cpk (based on Rbar) in capability analysis when using individuals (rolling estimate of Rbar is typically used).
    The idea of long/short term variation has always seemed alien to me. Hasen’t the long term process drift that Motorolla experienced been refered to as “seasonal” variation or something like that (i.e. assignable cause).Β  Did they do this to say they had Cpk=2 when they were really short due to this cyclical variation?
    Another note of interest, I don’t think ASQ hasΒ any mention of long term and short term variation in its CQE BOK.Β  I would think that short term would equal “machine capability” and long term would equal “process capability”, but that is too easy…
    Β 
    From Minitab:
    Description
    What do “within” and “overall” standard deviation mean in Capability
    Analysis and Sixpack (Normal)?
    Solution
    Within and overall refer to different ways of estimating process variation. A within estimate, such as Rbar/d2, is based on variation within subgroups. The overall estimate is the overall standard deviation for the entire study. Cp and Cpk are listed under Potential (Within) Capability because they are calculated using the within estimate of variation. Pp and Ppk are listed under Overall Capability because they are calculated using the overall standard deviation of the study. The within variation corresponds to the inherent process variation defined in the Statistical Process Control (SPC) Reference Manual (Chrysler Corporation, Ford Motor Company, and General Motors Corporation. Copyright by A.I.A.G) while overall variation corresponds to the total process variation. Inherent process variation is due to common causes only. Total variation is due to both common and special causes. Cp and Cpk are called potential capability in MINITAB, because they reflect the potential that could be attained if all special causes were eliminated.

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    #83435

    ScottS
    Member

    jabavock and moly figured it out for me.Β  Thanks all.

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    #83431

    ScottS
    Member

    Thanks moly.Β  That makes sense…Β  After Ross’s message, that is where I would have headed on my own.Β  Thanks for the help.

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