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  • #42934

    Kara
    Participant

    I’m just starting my lovely path down Six Sigma and I need a little boost.
    I’m going through the Six Sigma Online course and working through the Supplemental Exercises.  In one slide, there is a table of Z values as they correlate to PPMst, Cpk, & PPMlt.
    I guess I have a mental block as to how to get the PPMlt value.  If I’m given Short Term values of Zst 3.5, PPMst 233, & Cpkst 1.2 — how do I actually calculate PPMlt?  I think I’ve deduced that it’s 22750, but the number doesn’t mean anything to me as I haven’t been able to figure out how it’s derived.
    Any help is greatly appreciated!
    Thanks!

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    #135777

    Kara
    Participant

    Actually, I guess I need to know the ST too…is this just something we look up in a table or is this something I can sit down and calculate?
    Thanks!

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    #135779

    Vinay Goyal
    Member

    Can you show the steps how did you come up with this number?

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    #135793

    Kara
    Participant

    The Z, PPMst, & Cpk were given to me by the exercise.  I found 22750 in a table from the web where the Z was 3.5 and Cpk was 1.17 (1.2 rounded).  (http://www.westgard.com/sixsigtable.html).
    I can look up things in a table, but I imagine there’s some math behind it to get those numbers. :)

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    #135795

    Zero_Cool
    Member

    Base on your post, you know how to calculate the Zst, PPMst, Cpkst You know the formulae and how to use them.
    To calculate Long term, just Subtract 1.5 to your Zst thus Zlt = 2, then you get the 22750.  
    Zlt = Zst – 1.5
    Zero_Cool

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    #135823

    TwoCents
    Member

    “Zlt = Zst – 1.5”
    This is the “Six Sigma” answer…and probably the one that your course is looking for.  It is an estimate based on “empirical evidence compiled by the early adopters of Six Sigma based on on 34 years of statistical data…*blah, blah, blah*.
    That being said, there are many “doubters” (myself included) that question whether this actually holds any water in “real life”.  The assumption is that your process will be worse in the long term, because of “shift and drift” effects in the process.  The concept itself makes sense, it’s the 1.5 that people tend to have a problem with.  In fact, many practicioners have thrown this out the window.
    My take:  If you want long term estimates, take long term data.  In the mean time, don’t assume that your process will be “as good as it is at the moment” over the long term.  I also tend to believe that this “shift” will vary depending on where you are on the “curve”.  Keep in mind:  a 1.5 Sigma shift at 3 Sigma is a whole lot different than a 1.5 shift at 6 Sigma.
    For example:  By this calculation, if your short term capability is in fact 6 Sigma (2 defects per billion opportunities), then your long term will be about 4.5 Sigma (about 3.4 defects per million opportunities).  Ok, that seems reasonable…I can see picking up some defects here from shift and drift.
    But if you short term capability is 3 Sigma (1350 DPMO), then by this calculation your long term capability will be 1.5 Sigma…a whopping 67,000 DPMO!  That I have a little harder time with…
    This whole discussion is one of the “trees” that a lot of people have missed the “forest” for…
    So your best bet is to ignore everything I just said.  Just be careful…make sure you know the difference, and more importantly how you, your organization, and the people you talk to are coming up with these numbers.  Ultimately, IMHO, it doesn’t matter whether you use the shift or not, as long as you’re consistent.
    (By the way, there are about a million threads on this topic on this forum…I hope I didn’t just start another one…)

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    #135878

    Zero_Cool
    Member

    Huh…
    Was I hit by my own light saber accidentally or what? Probably, I need one of those vader mobile to take me to Death Star for immediate surgery after that lazy post of mine.   
    Two Cents,
    The question need an answer eventhough I never believe in 1.5 sigma shift myself. Go back to our newbie question and hold your breath by lecturing me about this dark side of yours for we have same cause. Lord Vader will give you a serious spanking for this.
    Zero_Cool
     
     

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    #135880

    TwoCents
    Member

    LOL!
    No worries, Zero Cool.  I wasn’t intending lecture you or to reprimand your “lazy post”.  I just saw the question coming…”Well where does 1.5 come from?” and figured I’d nip it in the bud.
    I’m with ya, brother…

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