Defining Six Sigma

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    Michael W McLean

    A comment on your definition: “Six Sigma quality” was as you stated , “the design tolerance should be 6 sigma – or twice as large as the natural tolerance” was confusing and somewhat at odds to Doctor’s Shewart, Deming & Wheeler. But Harry & Schroeder state in their book that “Six Sigma provides specific methods to re-create the process so that defects & errors never arise in the first place”. This supports that garage and car analogy used in some Six Sigma texts. 
     The calculation of Std Dev’n or Sigma was as others found back in the 20’s with Shewart et al, but the issue of the Control Chart (Voice of the Process) for stability first BEFORE CAPBILITY i.e. the Histogram (Voice of the Customer) has the +/-3 Sigma built in. Is the ASQ’s CQE/M exam giong to change as a result of H&S/Motorola definition or is SPC Charting calculations going to be changed – I don’t think so. 
     H&S go on to say that the “Process is not always centered so Motorola gives a +/-1.5 Std. Dev’n. drift allowance” (see Pyzdek 2001 too) to widen the tolerance so to speak i.e. the garage.  This takes the calculation back to the Shewhart origins of +/-3 Sigma anyway. Cpk for Aero/Auto etc industries is still the how close the Process Limit is to the nearest Spec/Tolerance, is Boeing and GM going to re-engineer their processes to allow for this “Drift”. 
    I wonder how the good Doctors would see this “process re-creation” and look forward to comments from others. There appears a mixed message out there based around unusual math and M Hammers’s recent MIT Sloan Paper on Six Sigma points to other problems in Six Sigma land.


    Mike Carnell

    Your post seems to reflect your confusion pretty well.
    In most instances the tolerances are not a function of what the process can yield but what the design needs. If it isn’t critical to the design then why would it cause anyone an issue to open the tolerance up? Even at that point why whould it open up to what the process is delivering? That isn’t any more intelligent than using the default tolerance block on the drawing.
    As far as what is on the ASQ exam. It hasn’t got anything to do with the job a company hires you to do. It is a test not a job.
    An off center process. You put 6 std deviations between your closest spec and the mean. If it doesn’t fit you center it and work on reducing the variation and shift the mean as you reduce the standard deviation. This isn’t all that difficult.
    You can probably reduce your confusion if you take Hammer, Harry and Schroeder off your reading list. It is like expecting to gain insight into football listening to Dennis Miller. They are entertainment. If you want information you listen to Madden – at least he has played the game.
    If you are looking for that intuitive level of understanding you read a little less and get a project. Once you actually have to fix something the authors and tests will cause you a little less stress because you fixed something and everybody elses opinion is irrelevant.


    Michael McLean

    Mike Carnell,
    Thanks for the reply and advice about reducing my confusion about the Six Sigma terms. 
    However, in the many Six Sigma projects I have completed, in supplying Six Sigma tools to the like of GE Capital, HP and Allied Signal plus training the Six Sigma fraternity of Master BB etc; the ASQ – CQE Graduates and Professors of Statistics and Quality, they have raised the questions I asked about i.e.:
    1. The definition of Sigma in “Six Sigma” as it differs from the standard definition and from those of  Drs Deming, Wheeler, Ishikawa and Shewhart
    2. Finally, I sought clarification on the Six Sigma Allowance of  +/-1.5 Std Dev Processs drift, has yet to be seriously answered. Would appreciate a response to the specifics.
    Michael W. McLean

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