Minitab and JMP: different results for CPK in Lognormal data

[#1]

Dears,

in this moment I'm evaluating a set of data.

This data are lognormal distributed.

I perform the CPK analysis wiht JMP and with Minitab: the capability resutl is different.

Some one of you, knonw the reason?

I think it could be linked to the type of formula usage for capability calculation... but I'm not sure.

Thanks in advance for the feedback.

Best regards,

Simone

[#2]

Have you verified in the JMP and Minitab documentation, that the formula used for the calculation is supposed to be the same?  Or do they have different ideas on what the calculation should be?

[#3]

Hello Nelson,

I'm trying to do this.

To be honest I was not able to find the formua used in JMP.

Do you have some suggestion about where I can found it?

Thansk in advance.

Best Regards,

Simone

[#4]

What spec values are you using? What are the values of the capability indices reported by Minitab?

[#5]

Dear David,

in attachment you can find the data used for this analysis.

You can also see the comparisono between Minitab and JMP on the same data set.

Thanks in advance for your feedback.

Best Regards,

Simone

[#6 · Accepted solution]

Hi Simone

Thanks for sharing the Minitab results.  

JMP has two places where you can perform process capability. The Distribution platform and the Process Capability platform (under Analyze>Quality and Process).

You've used the distribution platform, which is the platform that I prefer to use.  However, the Process Capability platform provides additional options for computing capability indices for non-normal distributions (note to self: this seems to be true also for v15 of JMP which has had process capability overhauled in the distribution platform.

The key point is this.  In the Process Capability platform you can two different types of calculation:

Switching to Z-score gives the same result as Minitab.

This hopefully explains the difference - although begs the bigger question of which method to use (coincidentally I am currently investigating this same question but as of yet I'm not in a position to express an opinion - having said that, if you transform the data the result for this data is comparable to the Z-score result).

[#7]

Dear David,

Thanks for the GOOD Explanation!

I followed your suggestion also for other type of distribution (like Weibull).

There is a difference if I use Percentiles or Z-score (and with Z-score the results matchs with Minitab).

This is good... for the understanding: step 1.

Now coud be usefull to understand the differences of CPK resutls... according the difference in the "Capability Index Method": as you can image... in front of a discussion (with a customer or supplier)... this can create long long additional discussions.

Thanks.

Best Regards,

Simone 

[#8 · Accepted solution]

Hi Simone,

You can find the documentation about the two different methods for calculating nonnormal capability indices here.  I hope that helps some.  Also, starting with JMP 15, both of these methods for calculating nonnormal capability are available in the distribution platform as well for nonnormal distribution fits.

Laura

[#9]

Dear Laura,

thanks for the details and feedback.

All is clear.

Have you a good day.

Best Regards,

Simone

[#10]

I also have a strong interest in understanding the differences.  I'm currently doing some investigations into this area - I'll let you know when I've written them up.