I'm migrating 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 on Windows XP SP2 (details below), lmer() from Matrix 0.995-8, and lme() from nlme 3.1-68.1 Thank you in advance for your help. Sincerely, Jon Olav Vik > library(nlme) > summary(lme(travel~1,data=Rail,random=~1|Rail)) Linear mixed-effects model fit by REML Data: Rail AIC BIC logLik 128.177 130.6766 -61.0885 Random effects: Formula: ~1 | Rail (Intercept) Residual StdDev: 24.80547 4.020779 Fixed effects: travel ~ 1 Value Std.Error DF t-value p-value (Intercept) 66.5 10.17104 12 6.538173 0 Standardized Within-Group Residuals: Min Q1 Med Q3 Max -1.61882658 -0.28217671 0.03569328 0.21955784 1.61437744 Number of Observations: 18 Number of Groups: 6 > library(Matrix) > lmer(travel~1+(1|Rail),data=Rail) Linear mixed-effects model fit by REML Formula: travel ~ 1 + (1 | Rail) Data: Rail AIC BIC logLik MLdeviance REMLdeviance 126.177 127.9577 -61.0885 128.6251 122.177 Random effects: Groups Name Variance Std.Dev. Rail (Intercept) 615.286 24.8050 Residual 16.167 4.0208 number of obs: 18, groups: Rail, 6 Fixed effects: Estimate Std. Error t value (Intercept) 66.500 10.171 6.5383 > version _ platform i386-pc-mingw32 arch i386 os mingw32 system i386, mingw32 status major 2 minor 2.1 year 2005 month 12 day 20 svn rev 36812 language R ______________________________________________ R-help@stat.math.ethz.ch mailing list https://stat.ethz.ch/mailman/listinfo/r-help PLEASE do read the posting guide! http://www.R-project.org/posting-guide.html