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Control charts upper and lower control limits calculation

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  • #53649

    Duivis
    Member

    According the Minitab help function, the upper and lower control charts in the standard setting are by default 3 times the standard deviation, however i recalculated this and the limits do not match this. What is the exact calculation? Used the function with an I-MR chart but this applies to all charts.

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    #190991

    Robert Butler
    Participant

    Without the numbers you used and a description of what you actually did there isn’t much of anything anyone can offer. If the data set is small enough copy/paste it in your next post and perhaps I or someone else could offer some suggestions.

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    #190992

    Duivis
    Member

    See attachment. [file name=Controlexample.xls size=69120]https://www.isixsigma.com/images/fbfiles/files/Controlexample.xls[/file]

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    #190993

    Mikel
    Member

    Rosita,

    Standard deviation is estimated as MR/d2. I ran the numbers you sent and the control limits are 3 * this estimate.

    Minitab is correct.

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    #190996

    Duivis
    Member

    I do not doubt that Minitab is correct, but how is MR/d2 calculated from these numbers?

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    #190997

    Robert Butler
    Participant

    An adequate discussion of the mechanics and the theory behind MR/d2 is beyond the scope of a discussion forum of this type. If you wish to understand the basic principles behind MR and d2 and the why of that term you will need to do some reading.

    You could start with Quality Control and Industrial Statistics 4th Edition – Duncan which has the discussions and, in the lookup tables for d2 (Tables D in the appendix), the citations of the original papers covering the derivation of d2. Another possibility would be to run a search on MR/d2 on the web.

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