"Anon." <[EMAIL PROTECTED]> writes: > I can only assume I'm betraying my ignorance here, but this is not > what I would expect. > > I'm getting the following from a stepwise selection (with both step > and stepAIC): > > > step(lm(sqrt(Grids)~ SE + Edge + NH), scope=~ (Edge + SE + NH)^2) > Start: AIC= 593.56 > sqrt(Grids) ~ SE + Edge + NH > > Df Sum of Sq RSS AIC > <none> 2147.0 593.6 > + Edge:NH 1 3.0 2143.9 595.1 > + SE:NH 4 23.2 2123.8 598.4 > - NH 1 75.8 2222.8 601.6 > - Edge 1 448.7 2595.7 646.4 > - SE 4 1033.7 3180.6 699.1 > > > My problem is that the SE:Edge term is not added. Now, I know that > I've specified the terms in a different order in the start model and > the scope, but this shouldn't matter, should it? Aren't these model > specifications commutative? > > Am I missing something important, or is this just an obscure feature?
Looks a bit odd, but the first thing I would do is to check whether the model with that term added actually makes sense: m1 <- lm(sqrt(Grids)~ SE + Edge + NH) m2 <- update(m1,~.+SE:Edge) anova(m1,m2) My hunch is that you'l find zero DF between the two models, but try it and see. -- O__ ---- Peter Dalgaard Blegdamsvej 3 c/ /'_ --- Dept. of Biostatistics 2200 Cph. N (*) \(*) -- University of Copenhagen Denmark Ph: (+45) 35327918 ~~~~~~~~~~ - ([EMAIL PROTECTED]) FAX: (+45) 35327907 ______________________________________________ [EMAIL PROTECTED] mailing list https://www.stat.math.ethz.ch/mailman/listinfo/r-help PLEASE do read the posting guide! http://www.R-project.org/posting-guide.html
