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...rating to lmer() from lme(). I have noticed that AIC values from lmer() are 2 units smaller than those reported by lme(). Could someone please explain why? The issue can be replicated with the first example from Pinheiro & Bates (2000) Mixed-effects models in S and S-plus. library(nlme) Rail2 <- Rail summary(lme(travel~1,data=Rail2,random=~1|Rail)) # AIC = 128.177 detach(package:nlme) library(Matrix) lmer(travel~1+(1|Rail),data=Rail2) # AIC = 126.177 Output is given below. The parameter estimates, standard errors and log likelihood seem virtually identical. I'm using R 2.2.1...