# Need Help on Cpk Calculation

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Viewing 9 posts - 1 through 9 (of 9 total)
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• #56087

Jan
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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#203009

Robert Butler
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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#203011

Leonardo
Guest

Cpl = 0.21
Cpu = 0.47
Therefore Cpk: 0.21

None of the answers above is valid

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#203012

Robert Butler
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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#203014

Mike Carnell
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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#203018

Chris Seider
Participant

What a strangely worded question.

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#203024

Mike Carnell
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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#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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#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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