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Cp/Cpk vs. Pp/Ppk?

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  • #48603

    New ATI
    Participant

    Please  explain  the  main  differences?
    When  to  use  each?
    thanks  and  regards

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    #164449

    Ward
    Participant
    #164454

    Dr. Scott
    Participant

    ATI,
    Look at the reading Pete recommended. Also, think of it this way:
    Cpk used to be called “short-term” capability, now it is more appropriately called “potential” by Minitab. The reason is how std. dev. is calculated. The std. dev. for Cpk uses the R-bar/d2 method.
    Ppk used to be called “long-term” capability, now it is more appropriately called “overall” by Minitab. Ppk uses the “standard” long for calculating the standard deviation, hence overall. It ignores between or “within” point to point variation (for the most part), and instead considers the overall standard deviation.
    Try this demonstration. Run a capability analysis on the following sets of data. The first is a simple random set of data, and the second is the exact same data but sorted.
    84.23680.61983.50777.15991.22368.94169.78094.82966.31165.18566.27182.47259.55266.65355.09384.10088.99274.08877.048101.31476.66673.67475.80689.21764.46886.64165.58882.49274.31978.62394.11075.44861.48257.15959.33080.07161.05059.04053.21878.73171.33878.72877.07184.17871.14777.88174.12776.685
    and,
    53.21855.09357.15959.04059.33059.55261.05061.48264.46865.18565.58866.27166.31166.65368.94169.78071.14771.33873.67474.08874.12774.31975.44875.80676.66676.68577.04877.07177.15977.88178.62378.72878.73180.07180.61982.47282.49283.50784.10084.17884.23686.64188.99289.21791.22394.11094.829101.314
    You will notice a huge difference in the two estimates even though the data are the same.
    Let me know if you have any more questions on this.
    Good Luck,
    Dr. Scott

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    #164456

    Ward
    Participant

    Also be advised that older versions of Minitab (not MTB 15) and some other SPC packages will adjust standard deviation overall by the the bias correction factor “c4” which lead to a slightly different statement of overall standard deviation in Minitab’s Graphical Summary and the standard deviation calculation in the Capability Analysis. Some users would be puzzled by why the two standard deviations were not exactly the same.

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    #164465

    New ATI
    Participant

    That is  excellent,thank you

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    #164467

    Dr. Scott
    Participant

    ATI,
    First, you are more than welcome. Second, my example is a much exaggerated reason of why process control should be checked before capability.
    Does that make sense?
    Regards,
    Dr. Scott

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    #164475

    Vader
    Member

    More drivel … “short term” and “long term” are meaningless.  Research the origins of these terms in Harry’s rubbish on the Z shift.

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    #164484

    New ATI
    Participant

    Thank You.
    I  believe  yo  can  be  a  good  teacher .
    I  believe  in  st & lt Sigma

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    #164498

    Dr. Scott
    Participant

    Vader,
    I agree which is why I said that the terms “potential” and “overall” were better.
    But the meaning of potential and overall is definitely not rubbish, particularly if you find large differences between the two.
    Regards,
    Dr. Scott

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    #164500

    New ATI
    Participant

    Agree
    It  is  becomuing  usual  to  attack  those  concepts for  “no good” reasons ,just  because  of  envy against mikel Harry.
    What  a  shame? 

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    #164501

    Dr. Scott
    Participant

    ATI,
    I wanted to add a couple of thoughts (clarifications) to avoid a misunderstanding. I disagree with the use of short and long term capability. I prefer to call them what they are, potential (or within) capability and overall capability.
    As far as short-term and long-term variation or sigma level goes, I don’t really buy into that; specifically as it relates to the use of the 1.5 shift adjustments when determining sigma levels.
    Thanks,
    Dr. Scott 

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