lm/model.matrix confusion (? bug)
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.
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