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Need Help With Control Limits Question

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  • #55795

    Amihai Pantz
    Participant

    Hiya all,
    It seems easy one…Im trying to use the median Chart x+-AR but I get wrong.
    my n=8, x=12.62, A(8)=0.373….what is my mistake???

    A set of data from a process has 8 readings per sample and 50 samples. The mean of the
    50 sample means is 12.62. The mean of the 50 ranges is 0.18.Find the control limits for a
    median chart.
    A. 12.52 and 12.72
    B. 12.54 and 12.70
    C. 0.02 and 0.33
    D. none of the above

    Thanks

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    #201780

    Strayer
    Participant

    The reason no one answered this is probably that we resist doing your homework. What do you get for your answer? How did you decide on that answer? Tell us that and maybe someone well explain.

    That said, this is the sort of exam question I despise. It may be an interesting intellectual exercise but it has little to do with actual practice. What you should know is under what circumstances you may prefer a control chart centered on the median over one centered on the mean.

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    #201781

    Amihai Pantz
    Participant

    Hi,
    I am trying to get ready for the six sigma black belt exam and found this question on one of the sites.
    Homework…well…this is something I did long long time ago..

    Do you think those kind of questions are not in the IASSC exam?
    I have no clue what questions I will have so I’m trying to answer all of them and all kinds of type.
    General ones, which refers to wide knowledge of the terms and how to use the tols and ones that are more calculative, like calculate the LCL/UCL, MEDIUM, MEAN, COLORATION, MAIN EFFECT etc.
    What do you think?

    I would really appreciate some help as I really(!!) can’t find a site that says “this is the sort of the question you will have” so I can focus..

    Thanks,
    Ami

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    #201783

    Chris Seider
    Participant

    https://www.qimacros.com/control-chart-formulas/x-median-r-chart-formula/

    One source for you to consider but did you check isixsigma.com ?

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    #201786

    Amihai Pantz
    Participant

    Hi,
    Thanks.
    Yes I did looked at isixsigma.com but didn’t fined an exam.
    Also the formula you gave me doesn’t gives the right result…that is what I am wondering about.

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    #201791

    Chris Seider
    Participant

    I wonder where the given A2 is a typo…if it’s .473 you get one of the answers. https://www.qualitydigest.com/dec98/html/spctool.html shows A2 would be in vicinity of 0.473

    I also find it weird they worded it “mean of the means” instead “mean of the medians”. It looks like a poorly put together question that might even be testing the use of significant digits since A and B are so close to each other and I calculate “quickly” somewhere in between the two.

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