For a pedagogical purpose, I was trying to show how the formula for a simple
regression line (~1+x) could be crossed with a factor (~1:group + x:group)
to fit separate regressions by group. For example:
set.seed(201108)
dat <- data.frame(x=1:15, y=1:15+rnorm(15),
group = sample(c('A','B'), size=15,
replace=TRUE))
m1 <- lm(y~ 1 + x, data=dat)
m2 <- lm(y ~ group + x:group, data=dat)
m3 <- lm(y ~ 1:group + x:group, data=dat)
m4 <- lm(y ~ 1 + x:group, data=dat)
The simple regression is model m1.
The usual way to write the by-group regression is model m2.
In model m3 was trying to be explicitly clear and interact "1+x" with
"group".
Looking only at the coefficients, it appears that model m3 is simplified to
model m4.
R> coef(m3)
(Intercept) groupA:x groupB:x
0.3775140 0.9213835 0.9879690
R> coef(m4)
(Intercept) x:groupA x:groupB
0.3775140 0.9213835 0.9879690
I wonder if anyone can shed some light on what R is doing with the "1:group"
term.
Kevin
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