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What do you call this capability indice?

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  • May 26, 2005 at 2:21 am #39488

    bradbea
    ★ 10 Years ★
    Participant

    In the companies I’ve been with there seems to be three capability metrics:
    1.  Cpk/Cp, a short-term estimate where sigma is estimated using a control chart
    2.  Ppk/Pp, a long-term estimate where sigma is estimated using all the data, s
    3.  “Short-term” Ppk/Pp, where the long-term calculation is used, yet it is estimated with short-term data (30 pieces of data) . . .it has a higher goal (e.g., qualification with suppliers) realizing that the longer term capability will be less
    What do you call #3 in your company, since it is not technically Ppk?

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    May 26, 2005 at 3:12 am #120216

    Nihar Senapati
    ★ 10 Years ★
    Participant

    hello:
    The # 3 is generally not found in most organizations. However, #2 is found  through two means;
    1. Estimating variability from the population
    2. Estimating variability from the sample of 30 or more.
    Please let me know if it helps.
    Thanks.
    Nihar
     
     

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    May 27, 2005 at 12:01 am #120299

    bradbea
    ★ 10 Years ★
    Participant

    I don’t understand your reply.  My point in posting the message was to convey that several of the companies I’ve worked for do indeed confuse short-term versus long-term capability. In fact, I have seen specs that call for a higher Ppk than Cpk, which doesn’t make sense.  The third, non-sensical category (using the Ppk formula, but relying on a small sample size from short-term data) is common, in my experience.
     
     

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    May 27, 2005 at 12:15 am #120300

    Nihar Senapati
    ★ 10 Years ★
    Participant

    What I meant in my earlier posting is that third category is seldom used as sometimes quality professionals instead of estimating the sigma for second category from a sample or population , they create this third category by doing a sample of 30 or more and estimating sigma from that sample
     
     

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