[R] Intercept in model formulae.
David Orme
d.orme at ic.ac.uk
Fri Aug 30 10:13:54 CEST 2002
Hi,
I'm trying to create a linear model for a dataset that has a breakpoint e.g.
# dummy dataset
x <- 1:20
y <- c(1:10,seq(10.5,15,0.5))
plot(x,y)
I've modelled this using the following formula:
temp <- lm(y ~ x*(x<=10)+x*(x>10))
I want to be able to omit the intercept (i.e. force the line through
zero) from the first of these segments (x<=10) so that I'm only
estimating one intercept and two slopes. I've tried including -1 in
the formula, folllowing the lm(y ~ x -1) syntax to omit the
intercept from lm(y ~x) but the linear model still returns an
estimate of the intercept of the first segment. I can't work out what
the syntax for the formula should be. Is it possible in one step or
do I have to fit the two lines as two models?
Any suggestions?
Thanks in advance,
David Orme
> R.version
_
platform ppc-apple-macos
arch ppc
os MacOS
system ppc, MacOS
status
major 1
minor 5.0
year 2002
month 04
day 29
language R
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