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Cpk with non-normal data

Apr 12, 2011 4:22 PM
(2795 views)

I have JMP 8.0 and read in the used manual that there is a function called "Spec Limits", which will tell me what kind of data I have, and then I can use it to calculate Cpk. Is this correct and how does it work?

4 REPLIES 4

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Re: Cpk with non-normal data

If you use the distribution platform and get a distribution of your response then go under the red triangle and utilize >continuous fit >all you will then get an assessment of the fit of your data. Once you do that you can go under the red triangle and choose capability analysis where you would dial in the type of distribution, the lower and upper specification limits and the target value. Once you do this you can compare the Cpk of your data versus the Cpk if you had assumed it to be normally distributed.

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Re: Cpk with non-normal data

Hi Lou,

Can yo confirm where I select the distribution..? I try with the check box or via the drop down menu but neither seems to impact the calculated Cpk numbers. I can see the fits for each distribution on the histogram (Weibull, normal, log etc.), but Cpk number is always the same.

Can yo confirm where I select the distribution..? I try with the check box or via the drop down menu but neither seems to impact the calculated Cpk numbers. I can see the fits for each distribution on the histogram (Weibull, normal, log etc.), but Cpk number is always the same.

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Re: Cpk with non-normal data

Open JMP Sample Data File = Big Class

Choose Analyze Response = "Weight"; Click OK

Red Triangle on Response; Scroll down to Continuous Fit> "All"

Scroll Down to output; Compare the fit (Log Normal is best)

Go To Red Triangle> Capability Analysis

Add Specification; LSL = 50, USL = 150

In Pull down window on dialog box choose "LogNormal"

Cpk calculated will us Log Normal distribution to calculate

Message was edited by: Lou V

Choose Analyze Response = "Weight"; Click OK

Red Triangle on Response; Scroll down to Continuous Fit> "All"

Scroll Down to output; Compare the fit (Log Normal is best)

Go To Red Triangle> Capability Analysis

Add Specification; LSL = 50, USL = 150

In Pull down window on dialog box choose "LogNormal"

Cpk calculated will us Log Normal distribution to calculate

Message was edited by: Lou V

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Re: Cpk with non-normal data

Thanks, that worked.!