# z value from Ppk

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• #38927

Joseph Banerjee
Participant

How to calculate the z value from Ppk.Is it right to use the same formulae as in Cpk .
z bench =(Cpk * 3 ) + 1.5

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#117235

Thaly
Member

Joseph,
You need to know your mean and sigma (and your spec limits) to calculate z.z = (USL – mean)/ sigma.z = (mean – LSL)/ sigma.If you know your Ppk and any one of your variables (mean or sigma) then you could calculate the other and use those to calculate Z.
Hope this helps.

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#117238

Joseph Banerjee
Participant

Thank you but this didnt help
I used a weibul distribution for a non normal data and got the ppk and now how to get the Zst or Zlt from Ppk value

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#117239

Thaly
Member

I’m not sure if you could get Z from Ppk alone.
May be you could try to transform your data and fit it to a normal distribution and see if it fits better.
Do you know what shape parameter you used? If you could calculate mean,(and then from Ppk and spec limits, you could calculate sigma) then you could calculate your z value.
do post it if you come across a better solution.

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#117261

Mikel
Member

what the heck is a z bench?

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