Need Help on Cpk Calculation

This topic contains 6 replies, has 4 voices, and was last updated by Mike Carnell 2 months ago.

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September 7, 2018 at 12:57 pm #706939

Jan
@janety0312
Reputation - 15
Rank - Aluminum
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
September 7, 2018 at 3:20 pm #706941

Robert Butler
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@rbutler
Reputation - 6466
Rank - Gold
So what number did you get? Show us your calculations and we can probably show you where you went astray.
September 10, 2018 at 2:19 pm #706953

Leonardo
The answer is
Cpl = 0.21
Cpu = 0.47
Therefore Cpk: 0.21
None of the answers above is valid
September 11, 2018 at 4:43 am #706956

Robert Butler
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@rbutler
Reputation - 6466
Rank - Gold
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.
September 11, 2018 at 9:09 am #706959

Mike Carnell
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@Mike-Carnell
Reputation - 4911
Rank - Titanium
@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.
September 11, 2018 at 1:22 pm #706967

Chris Seider
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@cseider
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What a strangely worded question.
September 12, 2018 at 2:00 pm #706979

Mike Carnell
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@Mike-Carnell
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@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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