Baris Demiral <demiral.007 <at> googlemail.com> writes: > > Hi folks, > > I am new to lme in R, and I have a question regarding to the effect of scale > function on the lme. When I use the function to scale and centre the levels > of the fixed effects (e.g., X and Y; both have two levels) and write them to > new columns: > ex: > dat$cX<-scale(as.numeric(dat$X),center = TRUE, scale = FALSE) > dat$cY<-scale(as.numeric(dat$Y),center = TRUE, scale = FALSE) > > and compare the lme of centred model ran on cX and cY with the non-centred > model run on X and Y: > > centred.model > <- lmer(quest.ACC~1+cX*cY+(1|Subject)+(1|SetNo),data=dat.Transfer,family='binomial') > non.centred.model<- > lmer(quest.ACC~1+X*Y+(1|Subject)+(1|SetNo),data=dat.Transfer,family='binomial') > > I find that the two models give very different results not only for the > intercept of the fixed effect effects (which I can understand), but also on > the variance of the fixed effect coefficients, leading to the huge > differences in some case (interactions emerge).
It's hard to say exactly without the data. However: it is *not* surprising that in a model with interactions the estimates of the fixed effects change when you center the variables. The meaning of the main-effect parameter of X is 'the effect per unit increase in X on the response variable, *when Y=0*', and vice versa. What *is* surprising is that the interaction term is different. I would have expected that the interaction would be identical between models (unless there are numerical issues going on that are solved by centering: are you getting any warnings?), but that the intercept and both fixed effects would differ. For simplicity's sake, what happens if you try this with glm, ignoring random effects? I would suggest that follow-ups might go to r-sig-mixed-mod...@r-project.org ______________________________________________ R-help@r-project.org mailing list https://stat.ethz.ch/mailman/listinfo/r-help PLEASE do read the posting guide http://www.R-project.org/posting-guide.html and provide commented, minimal, self-contained, reproducible code.
