[R] interaction terms in formula of lm or glm

[Christos Hatzis]

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: r-help-bounces at stat.math.ethz.ch
[mailto:r-help-bounces at stat.math.ethz.ch] On Behalf Of Luke
Sent: Monday, April 17, 2006 12:08 AM
To: R-help at stat.math.ethz.ch
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

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