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Need Help on Cpk Calculation

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  • September 7, 2018 at 12:57 pm #56087

    Jan
    ★ 20 Years ★
    Guest

    Could someone help me understand why Cpk is answer b? I couldn’t reach any of the below answers by applying the Cpk formula.

    Qs: The reported Cpk for a process with an average of 94 units, a spread of 22 units and upper and lower specification limits of 125 and 80 units would be?
    a) 0.64
    b) 1.27
    c) 2.12
    d) 1.84

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    September 7, 2018 at 3:20 pm #203009

    Robert Butler
    ★ 20 Years ★
    Participant

    So what number did you get? Show us your calculations and we can probably show you where you went astray.

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    September 10, 2018 at 2:19 pm #203011

    Leonardo
    ★ 20 Years ★
    Guest

    The answer is
    Cpl = 0.21
    Cpu = 0.47
    Therefore Cpk: 0.21

    None of the answers above is valid

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    September 11, 2018 at 4:43 am #203012

    Robert Butler
    ★ 20 Years ★
    Participant

    I haven’t run the calculation but if the answer is .21 then it looks like there is a typo in the multiple choice column – perhaps c) is supposed to be .212.

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    September 11, 2018 at 9:09 am #203014

    Mike Carnell
    ★ 20 Years ★
    Participant

    @janety0312 I got 1.29. This is poorly worded question but it is also very representative of the type of questions organizations ask on BB/GB cert tests. The question you need to ask yourself is what does a spread of 22 units mean. I took it that it meant +/- 6 standard deviations so you divide by 6. That means the standard deviation is 3.6. The mean is skewed towards the Lower Spec Limit so you are calculating Cpk to the LSL.

    94-80/3(3.6)

    14/10.8 = 1.296 or 1.3

    All that assumes that spread means +/- 6. If spead means +/- 3 then you are wasting your time doing a Cpk calculation and need to fix something.

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    September 11, 2018 at 1:22 pm #203018

    Chris Seider
    ★ 20 Years ★
    Participant

    What a strangely worded question.

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    September 12, 2018 at 2:00 pm #203024

    Mike Carnell
    ★ 20 Years ★
    Participant

    @cseider Half the issue on taking these certification exams is being able to figure out what they are asking.

    To that note I know a person who, in one day, passed 3 certification tests for BB and 1 for MBB.

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    July 8, 2019 at 7:12 pm #240323

    Ddelisle
    Participant

    Hi Mike-
    I was about to post a similar question regarding the Cpk calculation. I was hung up on the meaning of “spread”. It really is not clear in my reading material nor anything on-line except for your comment. Sure enough when I divide the “spread” by 6 to get the std. deviation, then I get the same answer. I am still not clear on the meaning of spread.. can you clarify?

    Thanks
    Derek

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    August 5, 2022 at 2:05 am #258045

    Theresa_5566
    Participant

    I Managed to figure this one out:

    USL = 125
    LSL = 80
    Avg = 94

    Spread = 6 sigma = 22

    CpK = minimum (USL-mu / 3sigma or mu – LSL / 3 sigma)
    Cpk = minimum (2.82 or 1.27)

    Hence answer is 1.27

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