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-tp14292188p14292188.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.
