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
- :
- Negative variances?

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

Apr 29, 2013 12:41 AM
(611 views)

I'm trying to fit a model to data. I add "Random" attribute to all my model effects. My problem is that I some times get negative variances in my output. How can this be?

Since variances cannot be negative, what does that tell me?

Is it an error in JMP or am I misunderstanding something?

Where can I access the exact mathematical formulas used by JMP?

BR

Jesper

Jesper

7 REPLIES

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

Apr 29, 2013 1:51 PM
(471 views)

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

May 6, 2013 3:18 AM
(471 views)

I don't see that. Since all repetitions occur for each device, the repetition should not be nested with device. That was the case if repetitions 0 to 3 occurred for on device while repetitions 4 to 6 occurred for another device? Or am I misunderstanding something?

In any case, negative variances should be a** mathematical impossibility** (they are averages of squared i.e. positive values). Why does JMP still return negative variances?

BR

Jesper

Jesper

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

May 6, 2013 4:10 AM
(471 views)

Uncheck the option "Unbounded Variance Components" to force all positive variance components. Negative variance components can occur if the model is inppropiate for the data or for small sample sizes (in relation to variances).

Check the book (found under Help menu) *Modeling and Multivariate Methods* page 116.

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

May 8, 2013 4:23 AM
(471 views)

Thank you. I will look into that. I any case you are saying that my model is likely not appropriate for my experimental setup?

BR

Jesper

Jesper

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

May 8, 2013 11:28 AM
(471 views)

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

May 6, 2013 7:05 AM
(471 views)

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

May 8, 2013 4:22 AM
(471 views)

Each repetition represents a repeated measurement on the same device. Any variation between repetitions on the same device is thus random (i.e. unaccounted for).

BR

Jesper

Jesper