iSixSigma

Control Charts

This topic contains 5 replies, has 3 voices, and was last updated by  Chris Seider 2 months ago.

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  • #56108

    Karim Mohsen
    Participant

    what is the difference between if the special cause appears in sample range and sample mean in control charts?

    I know that it they appears in sample mean that is mean that there is problem in equipment or the operator

    #203098

    Robert Butler
    Participant

    Special cause can impact either the mean or the variability or both and changes in operators and equipment are not limited to changes in mean – these variables (along with lots of other things) can impact variation as well.

    #203104

    Karim Mohsen
    Participant

    @rbutler

    more variables like what that may impact variation than operators and machines ?

    #203105

    Robert Butler
    Participant

    Practically anything you care to name that has anything to do with any aspect of the way the process functions – is this a homework question or are you actually looking at a process. If you are actually working on a process tell us about it and perhaps I or someone else could offer more specific details.

    #203108

    Jay Arthur

    If the range chart is out of control, it means the variation between subgroups is too unstable. I’ve seen changes in raw materials shift performance. And environmental changes (temperature, humidity, etc.)
    The X chart uses the R chart to calculate limits, so if the range chart is out of control, the X chart is unreliable.

    #203109

    Chris Seider
    Participant

    Interesting thought by J.A. that I don’t consider well stated.

    Hopefully you have lots of data (e.g. 20+ subgroups) for calculating your limits to keep from chasing false positives on special causes.

    However, don’t keep worry from keeping you from gathering info and acting on special causes or finding reasons for special causes.

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