Thanks, Christos. Another relevant question: If I want to include the interaction term consisting of V2 and V3 (they are numeric vectors), should I use:
y ~ 1 + V2:V3 or y ~ 1 + I(V2*V3) or both are good? -Luke On 4/17/06, Christos Hatzis <[EMAIL PROTECTED]> wrote: > > If you want the quadratic term, you need to pass it as an argument in > function I(): > > y ~ 1 + V1 + V3 + I(V3^2) > > This is documented in ?formula > > -Christos > > -----Original Message----- > From: [EMAIL PROTECTED] > [mailto:[EMAIL PROTECTED] On Behalf Of Luke > Sent: Monday, April 17, 2006 12:08 AM > To: [email protected] > Subject: [R] interaction terms in formula of lm or glm > > I would like to include pairwise interaction terms for lm(). For example, > I > want to include the quadratic term of variable "V3". > > > my.formula > y ~ 1 + V1 + V3 + V3:V3 > > > my.data > y V1 V2 V3 > 1 31 1 42 140 > 2 32 0 43 120 > 3 33 0 57 150 > 4 34 0 55 132 > > > foo <- lm(my.formula, data = my.data) > > > foo$coefficients > (Intercept) V1 V3 > 29.47368421 -2.15789474 0.02631579 > > Why do the foo coefficients not include V3:V3 ? > > I thought that the variable "V3:V3" has the values of squares of V3 > elements, that is, > V3:V3 > 140*140 > 120*120 > .... > Am I wrong? > > If I specify a fouth varaible "V4" with square values of V3, and the > formula > is y ~ 1 + V1 + V3 + V4, it seems that the foo will give me different in > and > out-sample predictions. > > -Luke > > [[alternative HTML version deleted]] > > ______________________________________________ > [email protected] mailing list > https://stat.ethz.ch/mailman/listinfo/r-help > PLEASE do read the posting guide! > http://www.R-project.org/posting-guide.html > > > [[alternative HTML version deleted]] ______________________________________________ [email protected] mailing list https://stat.ethz.ch/mailman/listinfo/r-help PLEASE do read the posting guide! http://www.R-project.org/posting-guide.html
