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Can Process Sigma be Too High?

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  • #30945

    Tony G
    Member

    Let’s say you have a company that manufactures electronic control systems.  The engineering drawings that the factory uses to build the systems sometimes contain errors (defects) such as incorrect wire nomenclature, wrong wire guage, parts don’t fit together as specified, etc.
    We calculated that on each page of the drawings there are (on average) 179 opportunities to make an error.  Each wire has to have the corect color, guage, type, start and end points, nomenclature, etc.
    Multiplying 179 by the number of pages in each design (which often number in the hundreds), and adding in the opportunities for errors on each ISO form, gave us the total opportunities for error in each design.  From this we calculated a DPMO for each design (D/O*10^6).
    Our observed DPMOs translate to process Sigma levels between 4 and 5.5.  That does not seem to pass the common sense test.  I don’t think we are actually that good.
    We may miss a few defects in our data collection (10 – 15 %) but I don’t see where this would account for such high process sigma values.
    The root of the “problem” seems to be that we have such a large number of opportuities for error in our calculations.  To make matters worse, the process owners have suggested that we should count every leter in a wire nomenclature as an opportunity for error.  This would make our process sigma levels even higher. 
    I know it seems odd to consider a high process sigma level as a problem.  However, when it seems unbelievable, we run into a credibility issue with the whole Six Sigma methodology.
    It could also be that a process sigma level of 6 is just not good enough for our process.
    Has anyone else had experience in calculating opportunites for error in engineering drawings?  What methods have worked for you to produce reasonable results?
    Thanks in advance for your comments.

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    #81211

    Erik L
    Participant

    Tony,
    A practice that I’ve used, which might help you, is to use the BOM and multiply the number of items in the BOM by 3.
    Regards,
    Erik 

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    #81212

    Fernie
    Participant

    Tony,
    Could it be possible that you guys are not identifying ALL the defects in the drawings??, Have you done an R&R study to determine the accuracy/reliability of your inspectors??
    Just my 2 cents… regards,
    Fernie

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    #81213

    Loehr
    Member

    Hi Tony,
    Even though the sigma quality level at the opportunity level is quite high, your process yield rate could be very low if there are a large number of opportunities, as in your situation.  Suppose the probability of an opportunity being good is .9999.  With 179 opportunities per page, a 100-page design has only a 17 pecent chance of being completely correct (take .9999 to the 17900 power).
    If the probability of an opportunity being good increases to .99999, then the chance of getting an error-free design jumps to 84 percent (.99999 raised to the 17900 power).
    Each opportunity must have a very high sigma level when there are a lot of opportunities to allow the process to have an acceptable yield rate.
    Bye
     

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    #81215

    Arthur
    Participant

    I don’t understand how some of you can carry this to such an extreme.  Fact: there is only one wire that you are supposed to use.  Defect probability is 1 Wrong Wire, 2 No wire, 3 Something that fit that is not a wire.  Why would you count 10 X infinity the number of wrong wires and or all of the objects in the world that are not wires that could fit?  If you have two terminals that both require a correct wire then the possibilities are Process 1 insert wire: 1. Wrong Wire, 2 No wire, 3 something that will fit that is not a wire.  Process 2 insert wire: 1. Wrong Wire, 2 No wire, 3 something that will fit that is not a wire.   Is SS supposed to be simular to brain sugery?

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