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Control Chart Constants: What Are They? How Are They Useful?

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  • April 14, 2020 at 7:41 am #247114

    BHARADWAAJ NALLAPPA
    Participant

    Hi everyone,

    This is my first post here. Loved reading all the different posts in this forum.

    I have a question on control chart constants. Can anyone explain what they are? How are they useful / practical in a six sigma project? Any useful links to resources will be much appreciated.

    Thanks,

    Bharath

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    April 17, 2020 at 6:37 am #247161

    David007
    Participant

    Here are good blog posts and video that explain d2 and d3 using simulation:

    https://andrewmilivojevich.com/d2-constant-d3-constant/

    https://andrewmilivojevich.com/range-statistics/

    As far as practical use, it means that your control charts will have statistically valid control limits, so you can properly distinguish between “in-control” and “out-of-control”.

    Disclaimer: I’m not affiliated with the blogger or Minitab

     

    • This reply was modified 11 months, 4 weeks ago by David007.
    • This reply was modified 11 months, 4 weeks ago by David007.
    • This reply was modified 11 months, 4 weeks ago by David007.
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    April 20, 2020 at 9:59 am #247219

    Rhb
    Participant

    Control charts are utilized to analyze an existing process in two ways.  First, to see if the data is normally distributed and doesn’t show abnormal trends or spikes.  Second, to see if the process has been stable over time.  Process Capability can not be accurately calculated without these two attributes.  They are also useful to track performance before, during and after a project to see step-change created due to project activity.

    • This reply was modified 11 months, 3 weeks ago by Rhb.
    • This reply was modified 11 months, 3 weeks ago by Rhb.
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    April 20, 2020 at 10:38 am #247224

    Robert Butler
    ★ 10 Years ★
    Participant

    Control charts don’t test for normal distribution and, while the standard calculation for process capability requires approximately normal data, it is also true that process capability can be determined for systems where the natural underlying distribution of the data is non-normal.  Chapter 8 in Bothe’s book Measuring Process Capability titled “Measuring Capability for Non-Normal Variable Data” has the details concerning the calculation.

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