Dear Simon, Thank you so much! Actually, it seems that Crawley's R book adopted the information from that earlier version and he discussed "by" in the context of that version. I will take a practice according to your suggestion. Thanks again! Jianghua
Simon Wood-4 wrote: > > The problem here is that the help page you are looking at appears to be > from > an earlier version of `mgcv' than you are using (it's from a version that > did > not support factor `by' variables). Take a look at ?gam.models for the > version that you are actually using. > > The reason that your models give the same fit is because ~z and ~z-1 > differ > only in the identifiability constraints used, when `z' is a factor (for > all > linear type models). > > As far as model reasonableness is concerned: it's a bit difficult to say > without knowing the context. The only thing that stands out is that you > are > using an isotropic `s' term for the interaction --- this is fine if > `byear' > and `FAFR' are really naturally on the same scale, but if not tensor > product > smooths (`te') may be preferable, as the are independent of the relative > scaling of the variables. For plot interpretability, I'd drop the `main > effect' smooths and just leave in the interaction. > > best, > Simon > > On Tuesday 05 May 2009 16:53, willow1980 wrote: >> I am a little bit confusing about the following help message on how to >> fit >> a GAM model with interaction between factor and smooth terms from >> http://rss.acs.unt.edu/Rdoc/library/mgcv/html/gam.models.html: >> “Sometimes models of the form: >> E(y)=b0+f(x)z >> need to be estimated (where f is a smooth function, as usual.) The >> appropriate formula is: >> y~z+s(x,by=z) >> - the by argument ensures that the smooth function gets multiplied by >> covariate z, but GAM smooths are centred (average value zero), so the z+ >> term is needed as well (f is being represented by a constant plus a >> centred >> smooth). If we'd wanted: >> E(y)=f(x)z >> then the appropriate formula would be: y~z+s(x,by=z)-1.” >> When I tried two scripts, I found they gave the same results. That is, >> the >> codes “y~z+s(x,by=z)” and “y~z+s(x,by=z)-1” gave the same results. The >> following is my result: >> ########################################################################### >> “anova(model1,model2,test="Chisq") >> Analysis of Deviance Table >> >> Model 1: FLBS ~ SES + s(FAFR, by = SES) + s(byear, by = SES) + s(FAFR, >> byear, by = SES) >> Model 2: FLBS ~ SES + s(FAFR, by = SES) + s(byear, by = SES) + s(FAFR, >> byear, by = SES) - 1 >> Resid. Df Resid. Dev Df Deviance P(>|Chi|) >> 1 1.2076e+03 1458.4 >> 2 1.2076e+03 1458.4 1.9099e-11 5.030e-10 2.074e-10” >> ########################################################################### >> Is this in conflict with above statement that “If we'd wanted: E(y)=f(x)z >> then the appropriate formula would be: y~z+s(x,by=z)-1.”? Also, if you >> are >> familiar with GAM modelling, please have a look at my modelling process. >> That is, I want to study how one factor together with two smooth terms >> will >> influence the response. In model2, I also fitted the interaction between >> two smooth terms, together with the interaction of this interaction with >> factor. Is model 2 reasonable? I find it is rather complicated to >> interpret >> the plot of model 2. >> Thank you very much for helping! > > -- >> Simon Wood, Mathematical Sciences, University of Bath, Bath, BA2 7AY UK >> +44 1225 386603 www.maths.bath.ac.uk/~sw283 > > ______________________________________________ > 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. > > -- View this message in context: http://www.nabble.com/A-question-about-using-%E2%80%9Cby%E2%80%9D-in-GAM-model-fitting-of-interaction-between-smooth-terms-and-factor-tp23390342p23407597.html Sent from the R help mailing list archive at Nabble.com. ______________________________________________ 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.
