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X bar and R charts

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  • October 11, 2006 at 2:00 pm #44858

    Flavius
    ★ 10 Years ★
    Participant

    Can anyone explains why in X bar and R charting, R charts work too? I know about X bars work due to cental limit theorem, however I do not understand how come samples R distribution is supposed to always follow a normal one and therefore same actions that apply to X bar charts apply to R charts?
    Thanks.
     
    Flavius

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    October 11, 2006 at 2:12 pm #144531

    Monk
    ★ 10 Years ★
    Participant

    X-bar provides insight on ‘between’ sample variation and
    R-chart provided insight on ‘within’ sample variation.
    Hope this explaination is enough.
    Monk

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    October 11, 2006 at 2:21 pm #144534

    Ken Feldman
    ★ 10 Years ★
    Participant

    I would suggest that you do a little reading of the work by Dr. Donald Wheeler.  He will dispel your assumption that Xbar R charts work because of the Central Limit Theorem.  That’s a bogus assumption.  Ranges do not follow a normal distribution.  The use of the WE rules should be applied to the Xbar chart.  Out of control signals are good for the R chart with any other rule very carefully applied if at all.

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    October 11, 2006 at 6:47 pm #144559

    Flavius
    ★ 10 Years ★
    Participant

    Darth, thank you for reply.
    I did not say that R charts as parts of X bar and R charts work because of central limit theorem. I only said about X bar charts work because of central theorm, at least this is written in ASQ CQE, CSSGB and other reference materials.
     Please read more carefully before answering.
    My question was if someone could explain why the distribution of samples ranges follow also a normal distribution in control charting and therefore the same out of control rules apply as for X bar.
    Thanks.
    Flavius
     

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    October 11, 2006 at 6:59 pm #144560

    lin
    ★ 10 Years ★
    Participant

    Flavious,
    Please re-read Darth’s reply and you will get your answer.  But I will repeat it.   The distribution of range samples DO NOT follow a normal distribution.  The zones test should not be applied to the range chart.  And please do follow his advise and read Dr. Wheeler.  He will explain why it is not the central limit theorem that makes the Xbar chart work.
    Bill

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    October 11, 2006 at 7:15 pm #144561

    Carl
    ★ 10 Years ★
    Participant

    How wonderful that people are starting to read Wheeler !!!  … the undeniable expert on SPC.
    Be aware of his views on Six Sigma … “the sooner one wakes up (that six sigma is rubbish) … the sooner one can begin to compete”

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    October 11, 2006 at 7:25 pm #144563

    Ken Feldman
    ★ 10 Years ★
    Participant

    Bill, thanks.  I must have used too many words and rambled a bit.  Your succinct explanation covered it well.

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    October 11, 2006 at 8:54 pm #144569

    walden
    ★ 10 Years ★
    Participant

    Carl,
    Unfortunately, I haven’t read Wheeler’s views on Six Sigma. I would definitely like to look into it though. Can you suggest any reference materials for me to read?
    As you stated, he is “the undeniable expert on SPC”. Although many may disagree, maybe he is correct up to a point “that Six Sigma is rubbish”. I have to see his reasoning for stating such.
    Chris

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    October 11, 2006 at 10:06 pm #144573

    Flavius
    ★ 10 Years ★
    Participant

    Guys, thank you all for posting, I appreciate you answers.
    Cheers.Flavius

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    October 12, 2006 at 5:15 pm #144634

    Dr. Mikel Harry
    ★ 20 Years ★
    Member

    R chart focuses on within sample variation
    X chart focuses on between sample variation
    Put focus first on R chart before the X chart

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    October 12, 2006 at 6:43 pm #144642

    tbatson
    ★ 10 Years ★
    Member

    Mikel,
    I am trying to develop a R Chart and X-Bar chart.  Do you know where I can get examples of good quality charts?
    Thanks,
     

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