why the number 6 and nothing else!!!!

Six Sigma – iSixSigma Forums Old Forums General why the number 6 and nothing else!!!!

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    i am a newbie to 6sigma.
    i just need to know in simple layman language why is the number 6 in six is the shift of 1.5 sigma related.
    also difference in short term and long term sigma.awaiting answers soon.
    thanx and regards
    jayesh john



    Do you go to office each day, ok lets assume that office time is 9:30 and you reach on one day at 9:40, 9:20 etc etc.
    No this shows that there is a process variation that means that you were not able to reach the office exactly day after day at 9:30 sharp.
    If you plot all these points on a graph over a period of time you will have a normal data, that means some of the time you have reached bit late and some time early though the average would be around 9:30. assuming no major delays takes place due to any external factors etc.
    now if you boss says that no coming before 9:15 and not after 9:50, hence you will identify in all days that you have come how many were beyond 9:50 (defects as per boss) as well before 9:15 (defects as per boss)
    with this current data lets assume your average was 9:30 and variation was 5 minutes. hence at 1 sigma you would be take all the time that you have come between 9:25 and 9:35, at 2 sigma at 9:20 and 9:40 and so on. but this shows that at 3 sigma you touch your customer limits hence you are able to put in only 3 standard deviations between mean and customer specifications.
    now when the process is at 6 sigma you are able to fit in 6 standard deviations. at 7 sigma 7 and so on. when you take 100 points you will get only 0.00034 points beyond customer specifications limits when the process of your coming to office is at 6 sigma. on 1 million it becomes 3.4. anything beyond 6 this 3.4 goes in decimals hence we take 6 which is 99.99966% error free closest to zero error, but you may take it forward and have 7 sigma process which will have 1 defect in a billion opportunities and so on and so forth.
    hope this helps

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