Whoops! Sorry, forgot my session stuff, just in case ... R version 2.6.1 RC (2007-11-22 r43520) i386-pc-mingw32
locale: LC_COLLATE=English_South Africa.1252;LC_CTYPE=English_South Africa.1252;LC_MONETARY=English_South Africa.1252;LC_NUMERIC=C;LC_TIME=English_South Africa.1252 attached base packages: [1] tcltk splines stats graphics grDevices utils datasets methods base other attached packages: [1] debug_1.1.0 mvbutils_1.1.1 MASS_7.2-38 sfsmisc_1.0-0 ade4_1.4-5 Design_2.1-1 [7] survival_2.34 Hmisc_3.4-3 loaded via a namespace (and not attached): [1] cluster_1.11.9 gamlss_1.7-0 grid_2.6.1 lattice_0.17-2 latticeExtra_0.3-1 [6] rcompgen_0.1-17 tools_2.6.1 Mark Difford wrote: > > Dear List-members, > > Hopefully someone will help through my confusion: > > In order to get the same coefficients as we get from the following > > ## > require (MASS) > summary ( lm(Gas ~ Insul/Temp - 1, data = whiteside) ) > > ...................... > > we need to do the following (if we use model.matrix to specify the model) > > ## > summary ( lm(Gas ~ model.matrix(~ Insul/Temp - 1) - 1, data = whiteside) ) > > ...................... > > That is, we need to take out "two intercepts." Is this "correct"? > Shouldn't lm check to see if an intercept has been requested as part of > the model formula? > > If I do > ## > summary(lm(as.formula(Gas ~ model.matrix (~ Insul/Temp-1, > data=whiteside)), data=whiteside)) > > ....................... > > we get a strange model. But the formula part of this model qualifies as a > valid formula > ## > as.formula(Gas ~ model.matrix (~ Insul/Temp-1, data=whiteside)) > > ---------------- > > just as if I were to write: lm(Gas ~ Insul/Temp - 1, data=whiteside) > > But we know that the _correct_ formula is the following > > ## > as.formula(Gas ~ model.matrix (~ Insul/Temp-1, data=whiteside) -1) > > ----------------- > > (Sorry, this is getting really long) --- So, my question/confusion comes > down to wanting to know why lm() doesn't check to see if an intercept has > been specified when the model has been specified using model.matrix. > > Regards, > Mark. > > -- View this message in context: http://www.nabble.com/lm-model.matrix-confusion-%28--bug%29-tp14292188p14292212.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.
