compound symmetry correlation structure in nlme
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)