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Compromise on Quality

Six Sigma – iSixSigma Forums Old Forums General Compromise on Quality

This topic contains 6 replies, has 5 voices, and was last updated by  Bill Fowlkes 10 years, 7 months ago.

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  • #51961

    Kaushik
    Participant

    Hi,
    If 6 sigma means that the limits of the deviation are +(-) 3 sigma and higher the sigma e.g.7,8 etc, lesser is the DPMO. But as the sigma levels goes higher, are we not increasing the tolerance limits of our proces, thus compromising on quality, to have a small DPMO?

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    #182077

    Tuguchi
    Member

    It is common mistake!
    It is +/- 6 Sigma ,not +/- 3 Sigma!

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    #182079

    CT
    Participant

    Hi
    I think you may have 6 Sigma and control limits (+,- 3 Standard Deviations) mixed up.
    Your Sigma score is calculated by your customers requirements in your cases I guess the tolerance level they demand. There are two ways to improve your DPMO increase your tolerances (Not Good) or reduce variation so more items are produced within these tolerances (Good).
    As you improve your process and reduce variation and get more into your customer specification limits, you’ll expect you control limits to reduce.
    Does this help?

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    #182097

    Kaushik
    Participant

    Hi,
    I am understanding the improvement in the process part, but when we say ‘reduce variation and get more into your customer specification limits’ do we mean reduce the deviation (which means variation). If that is the case, do we reduce the standard deviation (which means a lowered standard deviation and a lower value of sigma means a better process)?

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    #182153

    CT
    Participant

    Yes I mean that you will get a lower standard deviation. But if you have more items in your customer specification your Sigma score will go up.
    The symbol for Standard deviation is Sigma this is not the sigma in “6 Sigma”. Is this what is confusing you?

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    #182163

    Remi
    Participant

    Hai Arun,
    your confusion I have encountered many times in training.
    The basis lies in ‘bad’ communication. What happens is that people often say ‘sigma’ when they mean ‘sigma-level’.
    Sigma:

    Word in Greek language that describes their letter “s” or “S”.
    mathematics: it is the name of the standard deviation of a population (often estimated on the calculated st.dev from a dataset that is a sample of the population). St.Dev is one of the measures for variation, spread or deviation.
    a lazy alternative of ‘sigma-level’
    Sigma-level: the quality level of a process. Calculated from data on the process and the specs from the customer. It represents for a normal distribution the number of steps of size 1 Sigma between Mu and closest Spec OR the number of steps of size 1 Sigma between both Specs (reated to Cpk and Cp respectively).
    Tip: To determine Sigma you don’t need the customer specs, it is a property of a process; to determine ‘sigma-level’ you need the value of the sigma of the process and the specs of the customer.So if you are making products that you sell to different customers, you have only one Sigma but for each customer a Sigma-level.
    Hope this helped, Remi

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    #182177

    Bill Fowlkes
    Participant

    Remi,
    That was an excellent description.  Thanks.
    Bill

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