(Posted because private reply to sender was kicked back). Note that you said "centered **response** variables in your original original post, not centered **explanatory** variables, which is what (your misquote of) Bates and Pinheiro said. ** Big Difference!** If you don't know what the difference is, I strongly suggest you consult a statistician.
Centering of explanatory variables can sometimes help numerical accuracy, but otherwise produces an exactly equivalent linear model with a different intercept. Obviously, it changes NONlinear models. -- Bert Gunter Genentech Non-Clinical Statistics South San Francisco, CA "The business of the statistician is to catalyze the scientific learning process." - George E. P. Box > -----Original Message----- > From: [EMAIL PROTECTED] > [mailto:[EMAIL PROTECTED] On Behalf Of Martin Julien > Sent: Friday, January 14, 2005 1:44 PM > To: [email protected] > Subject: [R] Centered variables and mixed-model > > I work in biology and I use mixed-model for my data analysis > In a scientific paper, the author wrote: > "All continuous exploratory variables were centred on their > median value > prior to inclusion in the analysis (Pinheiro & Bates, 2000)." > They refer to the book "Mixed-effects model in S and S-Plus" > by Pinheiro et > Bates in 2000. > I feel a bit strange with that paper because I can't find in > the book why > they centred the variables on their median. > So I have two question: > First, is it correct to centred the variables on their median in a > mixed-model? > Second, why they do that? > Thank > Julien > > -----Message d'origine----- > De�: Liaw, Andy [mailto:[EMAIL PROTECTED] > Envoy�: 13 janvier 2005 19:26 > ��: 'Martin Julien'; [email protected] > Objet�: RE: [R] Lme and centered variables > > I'm by no means expert in such matter, but where did you get > the idea that > such manipulation is needed? If you center the response by > its mean, that's > the same as taking out the intercept. There's no reason to do that > beforehand. As to centering by median, that's a first for > me, and quite > frankly I don't see what it buys you. > > Andy > > > From: Martin Julien > > > > I want to fit a linear mixed-model with my data but I want to > > know if I have > > to center all my responses variables on their median and why > > I have to do > > it? > > > > Thank > > > > Julien Martin > > > > > > > > > > [[alternative HTML version deleted]] > > > > ______________________________________________ > > [email protected] mailing list > > https://stat.ethz.ch/mailman/listinfo/r-help > > PLEASE do read the posting guide! > > http://www.R-project.org/posting-guide.html > > > > > > > -------------------------------------------------------------- > -------------- > -- > Notice: This e-mail message, together with any > attachments,...{{dropped}} > > ______________________________________________ > [email protected] mailing list > https://stat.ethz.ch/mailman/listinfo/r-help > PLEASE do read the posting guide! > http://www.R-project.org/posting-guide.html > ______________________________________________ [email protected] mailing list https://stat.ethz.ch/mailman/listinfo/r-help PLEASE do read the posting guide! http://www.R-project.org/posting-guide.html
