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Sigma in SPC

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  • #37191

    ROSS
    Member

    Hi:
        When we do X-BAR R, and we know the control limite of  X-BAR is ¡Ï/¡Ð 3 Sigma, but what is the realy meaning of sigma on it?
    Tks!

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    #108926

    SK
    Member

    Hi,
    In Control Chart the limits are calculated based on the process standard deviation (sigma) value.
    Here it must be noted that this sixms is not the sigma which comes out of DPMO calculation.

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    #108927

    SK
    Member

    Hi,
    In Control Chart the limits are calculated based on the process standard deviation (sigma) value.
    Here it must be noted that this sigma is not the sigma which comes out of DPMO calculation.

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    #108930

    ROSS
    Member

    Hi SK:
       thanks for your feedback first, but pls go ahead, what is the sigma realy mean?

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    #108933

    dogsxxt
    Participant

    Variation from common causes.

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    #108936

    Process Engineer
    Participant

    The sigma in a control chart is the variation seen from the data set and is an estimated sigma value, using statistical contstants.
     

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    #108937

    dogsxxt
    Participant

    More precise is inherent variation within the sample.

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    #109072

    SK
    Member

    Hi Toni,
    I just wanted not to confuse the audience between the two sigma vaules.
    1) The sigma value from DPMO. This is same for all the processes i.e. if a process is producing 3.4 DPMO then then it is of six sigma capability. Similarly, if a process produces  679,672 DPMO, then the sigma capability is one sigma only.
    2) In case of control charts, it is the standard deviation of the process. This may be 0.01, 2, 3.9 etc.
    So in the first case the sigma level (Capabilty in terms of DPMO) will be a constant value for a particular level. But in the second case (process standard deviation) sigma value shall differ from sample to sample also. 
     

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