# Need Help on Cpk Calculation

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This topic contains 7 replies, has 5 voices, and was last updated by Ddelisle 4 months, 1 week ago.

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

Jan@janety0312**Include @janety0312 in your post and this person will**

be notified via email.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) 1.840September 7, 2018 at 3:20 pm #203009

Robert ButlerParticipant@rbutler**Include @rbutler in your post and this person will**

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

0September 10, 2018 at 2:19 pm #203011

LeonardoThe answer is

Cpl = 0.21

Cpu = 0.47

Therefore Cpk: 0.21None of the answers above is valid

0September 11, 2018 at 4:43 am #203012

Robert ButlerParticipant@rbutler**Include @rbutler in your post and this person will**

be notified via email.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.

0September 11, 2018 at 9:09 am #203014

Mike CarnellParticipant@Mike-Carnell**Include @Mike-Carnell in your post and this person will**

be notified via email.@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.

0September 11, 2018 at 1:22 pm #203018

Chris SeiderParticipant@cseider**Include @cseider in your post and this person will**

be notified via email.What a strangely worded question.

0September 12, 2018 at 2:00 pm #203024

Mike CarnellParticipant@Mike-Carnell**Include @Mike-Carnell in your post and this person will**

be notified via email.@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.

0July 8, 2019 at 7:12 pm #240323

DdelisleParticipant@Ddelisle**Include @Ddelisle in your post and this person will**

be notified via email.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

Derek0 - AuthorPosts

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