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A question about Sigma Level

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  • #43189

    Danny Ng
    Participant

    My company is going to use scorecard to monitor the performance of all its manufacturing sites. I am working in one of the manufacturing sites and we need to show our sigma level of our assembly processes. There are two processes for assembly our products, A & B. We have individual yield data for each process. We don’t have any detail defect count per defective part. In addition, different product styles have different number of parts on them. We only have overall yield but we don’t have detail breakdown of opportunities of defect per style. It is not possible for us to compute DPMO based on current available data. Our headquarter QA told me to get the sigma level for last 12 months in this way
    a. Get first time yield (FTY) for A and B processes
    b. DPU of A = -ln[FTY A] & DPU of B = -ln[FTY B]
    c. Total defect per unit  TDU = DPU A + DPU B
    d. DPMO =  TDU x 1,000,000 / Opportunities of defect
    e. Get long term sigma level from sigma level table by DPMO and then add 1.5 to get short term sigma level
    My questions are
    a. Is above way correct to get the sigma level based on the yield of the process ? It seems to me that the DPMO is only a “numerical value” but without any actual meaning for analyzing our processes. We can only make use of the defective analysis based on yield loss data for improvement.
    b. If above way is not correct, how to get sigma level based on yield information ?
    c. What’s the definition of opportunities of defect for an assembled parts ? Is it equal to numbers of part to be assembled together ?
    d. As we have difficulty to get the opportunities of defect, could I just use an average value for the opportunities of defect to calculate DPMO ?
    I would like to have someone to enlighten me on above issue. Thanks !
    Wai

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    #136721

    Adam
    Participant

    The easiest way that I can explain it, without knowing your process, is as follows, which is known as the rolled throughput yield concept (RTY):For every step in your process, you must know the percent of first pass yield (FPY). Lets say for simplistic reasoning that there were four steps in the process. Step one had a FPY of 95.5%, step two 97%, step three 94.4% and step four 97%. What you would do is multiply all of the FPYs together to receive your RTY. Therfore, the RTY for this process would be 84.4%, or 161,000 defects per million opportunities (DPMO) which would give you a sigma level of almost 3. 1 sigma = 31% or 697,000 DPMO
    2 sigma = 69.2% or 308,537 DPMO
    3 sigma = 93.3% or 66,807 DPMO
    4 sigma = 99.34% or 6,210 DPMO
    5 sigma = 99.976% or 233 DPMO
    6 sigma = 99.9997% or 3.4 DPMOA great program to invest in is called minitab and will most likely require some form of formal training to be completely understood. Also keep in mind that every product produced will have a different sigma level. Industry average is about 3-4 sigma. I am not sure if this helps you, as I am not an expert on six sigma, but this is how I was taught in the military in our green belt certification program.

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    #136742

    Danny Ng
    Participant

    Adam, thanks for your help ! I understand your approach but I am now not sure which is the most appropriate approach to calculate the sigma level. I cannot find any exact answer from internet or reference books ….

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    #136755

    April
    Participant

    Follow your QA for two reason :

    He is your boss;
    His calculations are based on “estimated yield = e^DPU” from a  poisson based defect distribution

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    #136762

    Mr IAM
    Participant

    I don’t think doing what somone tells you simply because they are your boss is the correct approach.  If you do not believe your boss is correct, you should do some research and offer alternatives or ask for clarification – not follow blindly.  Which is what the poster seems to be trying to do.
    – M

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    #136785

    April
    Participant

    There are 2 point in my answer, in point two there is the explanation on why his boss is right. Try to do an effort and read entire answer.
    If you have time, try to think and let me know if you haven’t never done what your boss want.

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