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PPM calcuation

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  • May 6, 2002 at 12:34 pm #29388

    Amit Puri
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    I am working with MIG welding process, and product which is a rear fork for two wheeler is made ot from various weldings.
    If I identified 30 types of defects at final inspection stage can be possiable in one unit of rear fork, does it mean that oppurtuniy is 37 defects per units.
    At final inspection during one day when the production was 5600, 120 nos of rar forks were rejected. and I got 1331 defects out of these 120 nos of rear fork
    I calculated through the calculator on this site and I go 1.96 sigma level. Please confirm and tell me about the correct calculation.
     

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    May 6, 2002 at 2:25 pm #75187

    RR Kunes
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    Member

    We must go into the definition of a defect versus a defective unit.
    The definition of opportunity I utilize is a measurable chance for a defect to occur.
    a defect is any event that does not meet the customer specification.
    A defective is a unit with one or more defects.
     
    Based on your write up I inferred that you may have one defect with multiple ways for that defect to occur (i.e. a poor weld can be catagorized in many ways but it is still just one defect).
     
    So without knowing more about your situation I hope this helps.

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    May 7, 2002 at 1:59 pm #75225

    Manu
    ★★★★★★★★★★
    Participant

    You need to look at this the way you would like to focus on the process improvement.
    If the issue is in terms of MIG welding only, treat it as one opportunity, in that case you can count as  MIG weld defect per defective part .
    In other case if you feel that apart from welding there could be other issues which may lead to defect, treat each as unique opportunities to make defect. In this case the total defects per unit (defective fork), would be more than one.

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    May 8, 2002 at 3:10 pm #75269

    ROSS
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    Member

    Amit,
    Note that I agree with RR Kunes that there is a difference between a defective unit and the number of defects per unit. With that said, here’s how I would tackle the situation.
    From the data you provided:
    Total Units: 5,600
    Opportunities Per Unit: 30 (not sure why you then stated 37 later in your original post — I’m going to stick with 30)
    Total Defects: 1,331 (note: not using the defectives/defective units, but the total number of defects which you are trying to eliminate)
    Calculator: 3.912 Process Sigma
    Anyone else agree?

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    May 9, 2002 at 11:54 am #75299

    RR Kunes
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    Member

    The Z value is contained within the equation I originally posted.
    FYI
    Ron

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    May 9, 2002 at 2:17 pm #75318

    Schuette
    ★★★★★★★★★★
    Participant

    Original message stated that numbers were from one day’s production, so this is short-term data.
    DPO = 1331/168,000 = .0079.  Z = 2.41 short-term.  Subtract 1.5 to get long-term, you end up with ZLT = 0.91.
    The calculator automatically puts the shift in – put in 0.0 for shift, you will get the same answer

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    May 9, 2002 at 3:19 pm #75327

    ROSS
    ★★★★★★★★★★
    Member

    Jim,
    Whoops…you caught me. I too mix up the short and long term every once in a while. You are completely correct that the data was most likely short-term data and not long-term. Therefore, it should not have the 1.5 sigma added as the calculator automatically does.
    As an aside, I see that iSixSigma created a new look for the calculator and improved the explanations — great job iSixSigma. You guys/gals rock!
    Tony

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    May 10, 2002 at 1:29 pm #75364

    Edwards
    ★★★★★★★★★★
    Participant

    Amit,This is the way I see it using the sigma calculator on this site.Sigma CalculatorEnter your process opportunities and defects and press the “Calculate” button.Switch To: Basic ModeUnitsOpportunities/UnitDefectsSigma ShiftCalculation Results DPUDPMODefects (%)Yield (%)Process Sigma© iSixSigma 2000-2002

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