Six Sigma Basics – process sigma and defects

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    Tony Hayward

    This is an old issue of defects against process sigma with and without process shift. Surely processes have moved on since in the 1970’s Motorola built in their 1.5 sigma process shift (non-linear anyway) and with electronic controls, planned maintenance and process control charts – I cannot see the relevance of such a large process shift.

    Therefore in principle, if we have a 4.65 sigma process and we can keep it under control, we achieve the magic figure of 3.4 defects per million.

    How can we ever prove that we have such small defect rates, what samples sizes do we really need – bearing in mind that if we take different samples on separate occasions we get different results – and how do we approach this mythical six sigma (almost absolute perfection – 2.3 defects per billion) target in reality.

    Comments will be extremely helpful.

    Many thanks – Tony.


    Roger Ellis

    The way I like to talk about this in the training programs I conduct for Green Belts and Black Belts is this: The goal is continuous improvement. The goal is to select, plan and execute continuous improvement projects that reduce the cost of (poor) quality and improve customer satisfaction. The goal is not 3.4 DPMO, nor is it any other arbitrary number. In fact, you may very well find that measuring in terms of Sigma level of quality is not the most appropriate way to measure. Personally, I find that the Sigma level metric creates a lot of confusion, especially when explaining the 1.5 mean shift. I suggest that you think in terms of metrics that are more meaningful and relate directly to bottom-line business performance and customer satisfaction, and then work on continuously improving your performance on those metrics. Also, beware of choosing projects where you achieve localized improvement on defect levels that never shows up on the bottom line. Theory of Constraints thinking will help greatly in selecting projects that will have a meaningful impact on profit and ROI. Regards, Roger Ellis, PMP, SSMBB.


    Big Al

    18 Sigma, 1.1 Sigma, 6 Sigma – all great stuff and a “good ” target.

    But …. and here’s the key question (in my opinion) … what does your business and its customers need?

    Is this clear and understood? Have we crunched the dollars to understand where the balance lies between awesome customer service (the highest agreed sigma level to which we will perform) and returning to our investors an agreed level of profitability?

    Without that, the whole sigma level debate becomes meaningless.

    My ten cents worth …..



    I have yet to see a Six Sigma Process.


    Doug Von Feldt

    I just taught this subject on Friday in a GB class I am teaching. The 1.5 shift is very confusing but everyone finally understood it and why it was used. What I try to focus on is that in this case Six Sigma is just a measurement and it really doesn’t matter as long as you are consistant.

    I do agree with the notion that processes do tend to shift over the long term and it does make sense to look at processes in the short term and long term

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