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Control chart – X-bar R chart

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  • #51620

    d
    Participant

    Hi,
    What does it mean when the X-bar chart is in control and the R chart seems to be trending upward?
    Thanks,
    Pam

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    #179417

    Darth
    Participant

    Let’s start with the basics. The R chart measures within sample variation and the Xbar chart measures between sample variation. The R value that you plot represents the range or variation of your sample. The Xbar chart measures the mean of that sample. Thus if the Xbar chart is stable then the variation of the means is not changing over time. But, if the R chart is trending up then the range or variation of the sample is increasing and your process variation might be increasing although the mean is staying about the same. That increased variation will affect a recomputation of the control limits. First check the Western Electric rules to see if it is indeed trending or whether it is just common cause variation. Then investigate whether the process variation is changing or whether it might be your sampling scheme. Be sure you are getting rational subgroups. Does that answer your question?

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    #179451

    d
    Participant

    Thanks Darth. This was helpful.

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    #179455

    Severino
    Participant

    If your R chart is trending upward it may also mean you have a pirate in your subgroups…
     
    … sorry that was bad!  I hate myself now.  :-(

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    #179585

    Tinoco
    Participant

    Variation in Rbar chart means  variables affecting standard deviation are  changing.  Variation in X bar is less serious. might be alignment. Variation in Rbar more serious, like wheel coming off.

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