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4.5 sigma Control Limits

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  • #45279

    holein1
    Participant

     I am familiar the 4.5/6 sigma concept, but if I generate a control chart from short term data with +/- “3 sigma” control limits can someone explain why I cannot add the 1.5 sigma shift and produce along term “monitoring” type chart with control limits at +/- “4.5 sigma”
    Thanks

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    #147460

    qualitycolorado
    Participant

    Holein1- Hello!Regarding your question — I think you will find the answer at these iSixSigma.com links (quick explanations of the 1.5-signa shift and short-term vs. long-term):https://www.isixsigma.com/library/content/c010701a.asphttps://www.isixsigma.com/library/content/c010311a.asp… hope this helps …Best regards,QualityColorado

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    #147465

    BB
    Participant

    QualityColorado,
    Wake up sleeping beauty.  Nobody here believes that 1.5 crap.  Even Harry changed his tune on the 1.5 shift about 4 years ago.

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    #147466

    Ang
    Participant

    BB is correct but a little blunt.  Here’s the full history in brief:

    Harry confuses 1.5 Shainin Pre Control green band limits with control chart limits.
    Bill Smith observes “sudden shifts” due to special causes and broadens tolerances to Cp=2.
    Mikel Harry derives +/-1.5 as a “shift” in the process mean, based on tolerances in stacks of disks. This is referred to as the Z shift.
    Harry realises his error and says the 1.5 “is not needed”.
    Harry in about 2003 makes a new derivation of 1.5 based on errors in the estimation of sigma from sample standard deviations. For a special case of 30 points, p=.95 he multiplies Chi square factor by 3, subtracts 3 and gets “1.5”. Actual factor ranges from 1 to 18. He now calls this a “correction”, not a “shift”.
    Reigle adds a new calculation he calls a “dynamic mean off-set.”: 3 / sqrt( n ) where 3 is the value for control limits and n is the subgroup size. For n=4 he gets “1.5”. Reigle says “This means that the classic Xbar chart can only detect a 1.5 sigma shift (or larger) in the process mean when subgroup size is 4”.  Reigle is quite incorrect. Such data is available from ARL (Average Run Length) plots.
    In other words, there is no 1.5.  There is no 3.4.  Six sigma tables are nonsense.  Forget about 1.5.
    Control limits are always +/-3 sigma.
     

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    #147467

    qualitycolorado
    Participant

    Holein1,As BB points out in his reply to your post, there is considerable debate regarding whether the 1.5-sigma shift is useful (or even advisable to use). It is definitely controversial.Dr. Don Wheeler, a well-respected statistician, calls the Six Sigma shift “goofy” (see: http://en.wikipedia.org/wiki/Six_Sigma). More of Dr. Dr. Wheeler’s comments can be found in the free white paper from his site: http://www.spcpress.com/ink_pdfs/The%20Final%206%20Sigma%20Zone.pdf.Dr. Tom Pyzdek, another well-respected member of the Six Sigma community, has some interesting thoughts about the shift — see this article: http://www.qualitydigest.com/may01/html/sixsigma.htmlAs Dr. Pyzdek note, ” The 1.5 sigma shift is simply a correction that accounts for factors not included in our model of reality”. Whether the 1.5 sigma shift is something that helps you model your reality is something you will need to determine.Nevertheless, many Six Sigma courses still teach it, and ASQ’s “Body of Knowledge” also still refers to it (http://www.asq.org/certification/six-sigma-green-belt/bok.html) so, you may need to know how to deal with it in your control charting and certification testing…. hope this helps …Best regards,Quality Colorado

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    #147470

    Bash Me Too
    Participant

    BB:
    In what publication did Harry “confuse” the 1.5 shift with Shannin rules, or is this just more of your “feelings”?  I would like to read that report.
    In what publication did Bill Smith provide his reasoning of “sudden shifts.”  I would also like to read this report.
    I recall reading where Harry said the 1.5 shift “is not needed,” but you did not provide the entire quote, only that portion you want to post, perhaps to make a hollow point.  Why don’t you post the entire quote, not just what you want people to see?
    Seems you are taking things out of context from what was originally published.  This is the way of bashers.  They love to misquote people for personal reasons, usually related to ego building or jealousy.

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    #147471

    Bash Me Too
    Participant

    Quality Colorado:
    Great response!!!

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    #147476

    mand
    Member

    The 1.5 is pure snake oil.  It doesn’t exist.
    If you think it does, prove it !!!!!!!
    Perhaps you might use one of Harry’s so called “studies” … without references …

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    #147480

    Marlon Brando
    Participant

    Agree

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    #147481

    Marlon Brando
    Participant

    Congratulation  for  being able  to  bring  references and  apprropriate  articles to  support your  point  of  view.

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    #147482

    BB
    Participant

    Pyzdek’s article is a farce.  He pulls 1.5 out of the air without any justification whatsoever.  He still calls it a “shift” instead of a “correction”.
    If the 1.5 were really a shift it would mean that every process in America was out of statistical control at least 13-14% of the time.  With the idiots that are running six sigma programs, such a distasterous situation is becoming increasingly likely.
    My personal view is that the 1.5 is part of a Japanese plot to destroy American industry.

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    #147484

    holein1
    Participant

    Thanks everyone. Much appreciarted

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    #147485

    Marlon Brando
    Participant

    Really  very  funny?
    How  it  comes ,when  Japanese  has  nothing  to do  with  SS?
    They  are  satisfied  with  their TQC  & TPS??

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    #147486

    Marlon Brando
    Participant

    For  what?

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    #147487

    Howling Owl
    Participant

    With all respect, I think the error lies with some USA academics, statisticians, and quality experts, who’ve allowed this nonesense to go unchecked.
    Perhaps it’s an American plot to destroy American industry!
    This is typical of what happens when people only speak to those who agree with them, and have no time for anyone who disagrees! Isn’t this what happens on Capital Hill – no wonder this nation is in such a mess!

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    #147492

    Bash Me Too
    Participant

    Holein1:
    Here is another site reference on the 1.5 shift.  Looks pretty good to me.
    https://www.isixsigma.com/forum/ask_dr_harry.asp?ToDo=view&questId=18&catId=6

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    #147494

    B Westlake
    Participant

    Dear Thick as Two Planks,
    The Japanese don’t use SS because they know its a fraud. 

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    #147497

    qualitycolorado
    Participant

    Hole-in-1:You are welcome! Best wishes for every success on your Six Sigma journey.Best regards, QualityColorado

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