iSixSigma

Cp and Cpk

Viewing 5 posts - 1 through 5 (of 5 total)
  • Author
    Posts
  • #55679

    Brandon Thomas
    Guest

    I have a question, I am currently in my green belt course for CP&CPK (have actually been using Minitab17 to figure my plants sigma level for a year now) and learned a 6sigma level translates to 3.4ppm rejects or a 99.73% success rate. why at the 8 sigma level does it appear to translate to 64ppm rejects? if your sigma level improves I thought your rejects went down. can someone please help me

    0
    #201140

    MBBinWI
    Participant

    Brandon: Not sure how you are getting the values that you do for 8 sigma.

    But you may have stumbled upon “the dirty little secret” of the origins of Six Sigma. If you look at a z-table (a table of the standard normal distribution), you will find 3.4 actually relates to 4.5 standard deviations. 6 Sigma actually relates to 0.987 per billion. So why were you taught that 6 sigma equates to 3.4 per million? You see, someone early on decided that in the “long term” there was something referred to as a 1.5 sigma shift. This was supposed to account for shift/drift that causes more variation over long periods than was observed during “short term” periods. Most all data gathered was considered “short term”, as any data set of a continuing process must necessarily not include all data, thus there is a “longer term” that exists. With this background, it was determined that any process that was at 6 sigma short term, would shift/drift by 1.5 sigma, and so in the short term would only be at 4.5 sigma (3.4 defects).

    You can search the site (and elsewhere) for the 1.5 sigma shift. There is quite a bit of discussion here and elsewhere regarding whether this truly exists or not. My perspective is that some amount of long-term shift/drift does occur, but that 1.5 sigma is not absolute. Thus, 3.4 dpmo for 6 sigma is fictitious.

    Hope this helps.

    0
    #201155

    Chris Seider
    Participant

    It’s not “dirty little secret”. ;) Just teasing…

    What scares me is I heard an MBA student supposedly quote a professor talking about a side subject regarding six sigma. The student said if all of the data is within +/- 3 standard deviations on an SPC chart, that means it’s a six sigma process. I shuddered and wanted to check if it was written down in some lecture notes but decided not to pursue! @MBBinWI I figured you’d love this quip.

    0
    #201164

    MBBinWI
    Participant

    @cseider – you mean it’s not? I guess I’ve been doing it wrong all these years ;-}

    0
    #201166

    Chris Seider
    Participant

    No, my friend… @MBBinWi

    glad to know ya! see, I’m talkin’ like a Texan! ;)

    0
Viewing 5 posts - 1 through 5 (of 5 total)

You must be logged in to reply to this topic.