Why Only 6 Sigma. Why Not 7 Sigma?

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    Titu John

    Why people always talk about Six Sigma only …why not beyond that ie 7,8…..12 sigma for that matter? Can any body out there clarify this?You can reply to me at [email protected]
    Titu John



    Well Titu,
    Let me try.
    As we know, Six Sigma itself is a stringent quality goal and specific defect rate (3.4 million defects per million parts or opportunities).
    This Six Sigma Level is almost always considered to be a “Goal” and organizations and individuals working in them strive to reach this stringent target.
    Remember, when we say 6 Sigma quality, we expect defect rate to be 3.4 per 1,000,000 or less.
    The guess is that the average sigma level of manufacturing industries is between 3 – 3.5 sigma.
    In various continents you find various levels. This is due to the fact that three types of variations (or variance or variability factors) strongly drive the sigma level of any company.
    Example: Let us assume you are making some ‘X’ Product. There are three types of variations affecting the quality of this product. These are:
    1. Unit-to-unit variations or variables – These are product characteristics or process variables.
    2. Usage Variations – These are environmental factors (environment in which the product is operated) like climate, location, mechanical, chemical stresses, vibration, etc.
    3. Noise Variation – This is most important among the three. This includes variations in the attitude of people, the behavioral aspects, rigor in process implementation. No wonder you will find high Sigma Level in Japanese companies. This is due to their nature of work and attitude, which nearly worships the work as god!
    Considering all the above variations in the industry, it is far more difficult to go beyond a certain sigma level. Most of the processes followed in the world do not go beyond certain limits. That is the fact. Due to the current exhibited capability of the processes followed by companies we are can expect them to produce 6210 to 66,807 (i.e., 4 – 3 sigma) defects per million. 
    Why not 7 sigma level?
    Lets compare 3, 4, 6, 7 sigma levels.
    Following table can give you some idea.

    Total opportunities or parts

    Defects @ 3 sigma

    Defects @ 4 sigma

    Defects @ 6 sigma

    Defects @ 7 sigma

    1 million or   1,000,000
























    In example above, for a company producing 100,000 parts, if there are 621 defective ones, then the company is at 4 sigma level and it may analyze its economic loss and take actions. If another company is producing same number of parts, but operating at 6 sigma level, then the defects here itself are almost zero (0.34). Third company at 7-sigma will produce 0.00212 defects per 100,000 parts.
    How many companies really do produce parts or deliver projects which are more than 100,000 in quantity practically? Need to think on this question, isn’t it?
    Secondly, what I have not mentioned in this explanation to you (since it is taught during the training only) is that when we say a process or product is at 6-sigma level, we are considering the possible shift of the process average. (Remember we had taken Normal distribution with 2 parameters – Mean and standard deviation during our session). This is due to fact that no one is PERFECT. Process Average tend to shift as time passes and above three variations keep changing.
    Imagine, if this average remains the SAME, then for a 6-sigma level process, we can expect JUST 0.001 defects million opportunities, instead of 3.4 defects per million opportunities.
    So, just imagine a 6-sigma process operated with perfection can give us 0.001 defects per million only. Do we want to go further than this?
    To reach reach 6 sigma itself  we need to fast act on no. 1), 2) and 3) of the above variations and see how much we could achieve. Is it possible for India? For some companies due to attitude it is difficult, for others, they consider the impact of change and fight shy of improving their processes. They are satisfied with what they have.
    A challenge isn’t it for such companies?
    Hope this explanation helps you.



    Six Sigma is really only Four and a half Sigma …



    Haha good point Andy. We need to actually shoot for 7.5 Sigma to achieve our 6 Sigma goals :)

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