topic Re: Confidence Intervals For Cpk in Discussions

Confidence Intervals For Cpk

lisamaley — Fri, 09 Jun 2023 00:57:13 GMT

I was trying to replicate the confidence intervals for Cpk in JMP. I found this page which details the formula:

https://www.jmp.com/support/help/en/17.0/#page/jmp/statistical-details-for-capability-analysis.shtml

This page says the equation is:

So this is the example I am trying to match,

since it is 95%, the lower confidence interval should be (32 is the sample sample size):

This equals, 6.075. JMP says it is 5.532. Am I doing something wrong or is this not the correct formula, why is it not matching?

When I use this calculation for Ppk, I get the correct answers, but not Cpk.

I am on macOS Monterey 12.6 and JMP Pro 16.0.0.

Lisa

Re: Confidence Intervals For Cpk

Dan_Obermiller — Tue, 15 Nov 2022 16:53:13 GMT

The formula looks correct, or at least close to others that can be found on the internet. Following your values, I get the same results as you both in JMP and with my hand calculations. I also tried several other examples and my hand calculations never match what JMP is providing. I think this might be something that you take to JMP Tech Support to see if they can determine what is going on.

Re: Confidence Intervals For Cpk

lisamaley — Tue, 15 Nov 2022 17:17:34 GMT

Will do. thank you!

Re: Confidence Intervals For Cpk

Mark_Bailey — Tue, 15 Nov 2022 19:15:52 GMT

You might also take a look at this page for more detail about how the CI is currently calculated. Your result at the top appears to be from a legacy platform that might not be up to date. The key difference is the use of DF that are not necessarily N-1.DF depends on how the sigma is determined. The developer is looking into this matter, too.

Re: Confidence Intervals For Cpk

lisamaley — Tue, 15 Nov 2022 23:21:40 GMT

Thanks. I tried the new calculation and am still getting different results. I forwarded an email to jmp support.

Re: Confidence Intervals For Cpk

Oren_Barilan — Fri, 16 May 2025 04:50:09 GMT

Are there any updates from JMP about this issue?

Thank.