[R-sig-ME] compound symmetry correlation structure in nlme

[David Atkins]

Rian--

I believe you would want to compare an lme() fit with random intercept 
to a gls() fit with compound symmetry (and hence, no random terms at all).

Sort of think Pinheiro and Bates make this comparison in their book, but 
I don't have it handy.

Hope that helps.

cheers, Dave


hello all,

I am analyzing some data with repeated measures on individuals, and want 
to test different correlation structures.
In Zuur et al (2007) they state "the inclusion of a random intercept in 
a GLM is imposing the compound symmetrical correlation structure, just 
as it did in the linear mixed model" (Chapter 13, p.323), which I 
understand as meaning that if a random intercept is included in a model, 
then the compound symmetry correlation structure is implied.

When I run the following two models (which are identical, except that in 
the second one I explicitly specify the correlation structure), I get 
the same estimates for the random and fixed effects, but the AIC and BIC 
values differ, because the second model has one more parameter (for the 
correlation structure).

global.ri.lme <- lme(Log.minH ~ site + f.year + cohort + emerg + emergSQ 
+ pri + priSQ
   + start.t + startSQ + sea + tidem + tided, data = SUSCforage, random 
= ~1|scoterID,
   method = "REML")

global.ri.cs1.lme <- lme(Log.minH ~ site + f.year + cohort + emerg + 
emergSQ + pri + priSQ
   + start.t + startSQ + sea + tidem + tided, data = SUSCforage, random 
= ~1|scoterID,
   correlation = corCompSymm(form = ~1|scoterID), method = "REML")

So, if I want to obtain an accurate AIC value, which should I use?

thank you,

Rian

-- 
Dave Atkins, PhD
Research Associate Professor
Department of Psychiatry and Behavioral Science
University of Washington
datkins at u.washington.edu

Center for the Study of Health and Risk Behaviors (CSHRB)		
1100 NE 45th Street, Suite 300 	
Seattle, WA  98105 	
206-616-3879 	
http://depts.washington.edu/cshrb/
(Mon-Wed)	

Center for Healthcare Improvement, for Addictions, Mental Illness,
   Medically Vulnerable Populations (CHAMMP)
325 9th Avenue, 2HH-15
Box 359911
Seattle, WA 98104
http://www.chammp.org
(Thurs)