[R] Intercept in model formulae.

David Orme d.orme at ic.ac.uk
Fri Aug 30 10:13:54 CEST 2002


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))

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
major    1
minor    5.0
year     2002
month    04
day      29
language R 
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