Stata The Stata listserver
[Date Prev][Date Next][Thread Prev][Thread Next][Date index][Thread index]

Re: st: GLM and ANOVA complaints

From   David Airey <>
Subject   Re: st: GLM and ANOVA complaints
Date   Sun, 28 Sep 2003 20:50:19 -0500

Joseph Coveney kindly notes:

A couple of quick observations: (1) David might have trouble with -manova-
since each mouse has 48 observations worth of covariances to estimate and only
eight records (animals) to do it with (and even with the full dataset of 20
strains and 38,400 observations--or 800 animals--it would still appear
problematic to estimate the 1100-1200 elements in the variance-covariance
This is good to know. I discussed the design with a couple scientists who didn't suggest my intended data set would be shy in terms of the number of animals for the effects I want estimated. I'll ask again with these points your have raised. Fortunately, my study is in the beginning of data collection, and I while it is too expensive with my resources to obtain more within strain data in terms of animals, I could simplify the design by dropping one within subject effect which is not obligatory, and I could treat another as a single variable time effect, leaving two within-subject factors.

(2) this might be why the -repeated()- option is having trouble, too,
since it's trying to estimate deviation from sphericity/compound symmetry in
the same vein, (3) the ANOVA table was completed and, if David assumes
sphericity (which he might have to do, given the number records/mice to
actually assess the assumption), then the analysis has completed as far it can--
and this might be true with any software package that estimates epsilons. (I
believe that SAS's PROC MIXED is able to partition the variance-covariance
matrix into blocks according to the within-subject factor so that fewer off-
diagnonal elements need to be estimated, and this might enable full-rank MANOVA
to be done here.)
This is also very good to know. If SAS rips through these examples with SAS MIXED but stumbles with GLM, that will be a good hint your are right.

Other than a typographical error in mine (a missing forward slash in one line
of the model specification), our models are fundamentally the same: both
David's and mine are repeated-measures (split-plot) analyses of variance; I
didn't include the -repeated()- option in my repeated-measures ANOVA because I
had only two levels for each within-subject factor, rendering the option
I forgot that; it was even pointed out to me in another context by Ken Higbee a while back. Thank you so much for taking an interest on this. I'll see if my computer handles your do file in 30 minutes.

I've never tried it, so I'm not sure whether a zero residual mean
square will affect the -repeated()- option's operation.
I'll check this too with your do file.

P.S. The post appears in monotype font in my browser.
Ah ha! It's a bug in Safari 1.0 then.

Thank you again.


*   For searches and help try:

© Copyright 1996–2021 StataCorp LLC   |   Terms of use   |   Privacy   |   Contact us   |   What's new   |   Site index