[R] Intercept in model formulae.

[David Orme]

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)) # where 10 is the breakpoint

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 separate models on subsets of the 
data?

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 
-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-
r-help mailing list -- Read http://www.ci.tuwien.ac.at/~hornik/R/R-FAQ.html
Send "info", "help", or "[un]subscribe"
(in the "body", not the subject !)  To: r-help-request at stat.math.ethz.ch
_._._._._._._._._._._._._._._._._._._._._._._._._._._._._._._._._._._._._._._._