turn on suggestions

Auto-suggest helps you quickly narrow down your search results by suggesting possible matches as you type.

Showing results for

- JMP User Community
- :
- Discussions
- :
- Log-normal process capability

Topic Options

- Subscribe to RSS Feed
- Mark Topic as New
- Mark Topic as Read
- Float this Topic for Current User
- Bookmark
- Subscribe
- Printer Friendly Page

- Mark as New
- Bookmark
- Subscribe
- Subscribe to RSS Feed
- Get Direct Link
- Email to a Friend
- Report Inappropriate Content

Feb 9, 2017 7:25 AM
(689 views)

Hi, I have a process that is log-normally distributed and has an upper spec limit, and I'd like to calculate a Cpk. I tried two approaches that gave me different answers:

- Save spec limit as column property for process. Plot distribution of process, then Continuous Fit --> LogNormal to get a Cpk (checking goodness-of-fit of course)
- Add new column. Set column formula as log(process), save column upper spec limit as log(upper spec limit). Plot distribution, get Cpk from capability analysis. Validate via Continuous Fit --> Normal and checking goodness-of-fit.

Which of these approaches would you recommend? Or could you suggest a better approach?

Many thanks,

Alex

3 REPLIES

- Mark as New
- Bookmark
- Subscribe
- Subscribe to RSS Feed
- Get Direct Link
- Email to a Friend
- Report Inappropriate Content

Feb 9, 2017 9:51 AM
(681 views)

I consider both of the techniques you specified as good vialble processes. However, the process that I would actually recommend is a very nice builtin capability of the Distribution Platform. If you run the Distribution Platform, you can then go to

Continuous Fit==>All

It will then evaluate your data to find which distribution(s) it fits. Once done, and if a Log Normal distribution is found to be the fit, you can go under "GLot" distribution and

Save Transform

It creates a new column using the determined Generalized Log algorithm

Jim

- Mark as New
- Bookmark
- Subscribe
- Subscribe to RSS Feed
- Get Direct Link
- Email to a Friend
- Report Inappropriate Content

Feb 10, 2017 3:45 AM
(666 views)

Thanks Jim. It's a bit frustrating that the LogNormal transform can't be saved in the same way - the GLog transform adds additional complexity, which is hard to justify and would be (even) harder to explain to my colleagues.

Any idea why I got different answers between my initial two approaches?

- Mark as New
- Bookmark
- Subscribe
- Subscribe to RSS Feed
- Get Direct Link
- Email to a Friend
- Report Inappropriate Content

Feb 10, 2017 5:07 AM
(663 views)

I understand your frustration. The question on why log normal does not have a Save Transform would have to be answered by the JMP developer.

Concerning the difference between the Cp/Cpk for the Distribution lognormal and your own transformed to log(x) and then calculate.......it appears that there are differences in the calculation of the PDF. I am taking that from the description within

Help==>Books==>Basic Analysis

But a more complete answer should be available from JMP Support

Jim