Dear Bill, I am not a lme-expert, but I believe the Pinheiro&Bates' book is rather clear here.
However you know that a lme model is, for instance fixed= y~x1+x2 and random=y~x1|group and you can fit it by ML or REML. If you are interested in testing for x2 by means the LRT (namely by comparing the models with and without x2) you have to fit them by ML. hope this helps, best, vito ----- Original Message ----- From: Bill Shipley <[EMAIL PROTECTED]> To: R help list <[EMAIL PROTECTED]> Sent: Tuesday, November 04, 2003 4:42 PM Subject: [R] help with lme() Hello. I am trying to determine whether I should be using ML or REML methods to estimate a linear mixed model. In the book by Pinheiro & Bates (Mixed-effects models in S and S-PLUS, page 76) they state that one difference between REML and ML is that « LME models with different fixed-effects structures fit using REML cannot be compared on the basis of their restricted likelihoods. In particular, likelihood ratio tests are not valid under these circumstances." I am not sure what "fixed-effects structures" means. Does it mean that, as long as the types of contrasts are the same between two models, they ARE comparable, but are NOT comparable if the types of contrasts are changes? Or rather, does it simply mean that one should use t or F tests for the fixed effects, and restrict the likelihood ratio tests to the random effects only if using REML? Bill Shipley Associate Editor, Ecology North American Editor, Annals of Botany Département de biologie, Université de Sherbrooke, Sherbrooke (Québec) J1K 2R1 CANADA [EMAIL PROTECTED] <http://callisto.si.usherb.ca:8080/bshipley/> http://callisto.si.usherb.ca:8080/bshipley/ [[alternative HTML version deleted]] ______________________________________________ [EMAIL PROTECTED] mailing list https://www.stat.math.ethz.ch/mailman/listinfo/r-help ______________________________________________ [EMAIL PROTECTED] mailing list https://www.stat.math.ethz.ch/mailman/listinfo/r-help