# Z Value and Process Capability

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- This topic has 7 replies, 1 voice, and was last updated 7 months ago by subhayu.

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- July 26, 2010 at 8:23 pm #53532
Hey Guys

Can anybody tell me the difference between the Z value and Cp, Cpk?

0July 26, 2010 at 11:33 pm #190507Cpk = 3 * z

0July 29, 2010 at 5:36 pm #190515

RivasParticipant@marcelorivas**Include @marcelorivas in your post and this person will**

be notified via email.I think that Z=3*cpk.

Z is sigma level. If the best cpk=2, sigma level = 60July 29, 2010 at 5:42 pm #190516

RivasParticipant@marcelorivas**Include @marcelorivas in your post and this person will**

be notified via email.Cp= (LSU – LSL) / 6stdv

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Cpku= (LSU – X)/3stdv

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Cpkl= (X-LSL)/3stdvCpk = the lesser of two

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0August 11, 2010 at 3:24 pm #190590

MBBinWIParticipant@MBBinWI**Include @MBBinWI in your post and this person will**

be notified via email.Marcelo caught you on that one Stan.

However, the answer was incomplete. Z and Cp are not related mathematically (nor is Cpk related to Cp mathematically). Only in the instance where your distribution is perfectly centered between the LSL and USL will Cp = Cpk = Z/3 and this is not a mathematical relation, rather a definitional relation (yes, Stan, I am well aware that you understand this, I’m pontificating for the masses).1August 11, 2010 at 3:51 pm #190593

FurgesonMember@t.-Furgeson**Include @t.-Furgeson in your post and this person will**

be notified via email.MBBinWI,

We have been looking for you. We need to know if you are Stan’s mentor or his legal guardian.

Please advise,

t.

0August 12, 2010 at 6:26 pm #190606

MBBinWIParticipant@MBBinWI**Include @MBBinWI in your post and this person will**

be notified via email.I could be a real smart alec and accept mentorship, but that would probably get me canned.

And you certainly aren’t going to pin guardianship on me.0June 20, 2020 at 4:34 am #248471

subhayuParticipant@subhayu**Include @subhayu in your post and this person will**

be notified via email.I have a follow up question how is it that Z/3 = Cp/Cpk if the process is centered? I understand if the process is centered Cp=Cpk and both values of Cpk will be the same. What I dont get is how will that be equal to Z/3 assuming the Z statistic is Z=[X-(mu)]/(sigma)

Will appreciate a response since I am due to take the CSSBB exam next month

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