[R] model formula
Peter Dalgaard
p.dalgaard at biostat.ku.dk
Tue Nov 18 20:07:43 CET 2003
Bill Simpson <William.Simpson at drdc-rddc.gc.ca> writes:
> I have continuous variables x, y, z. The plot of the data looks like this:
>
> y
> | z=1(o), 2(@), 3(#), 4(*)
> |
> |* * *
> |
> |
> |# # # #
> |
> |
> |@ @ @ @
> |
> | o
> | o
> | o
> | o
> |o
> ------------------------ x
> The correct model appears to be: if z==1, y~x+z; else y~z
> (y~z + z:x isn't it)
Not if z really is continuous...
> How can I express this model in lm()? If I can't express it properly in
> lm(), what is the best way to fit the model?
I'd try something like
x2 <- ifelse(z==1, x, 0)
z2 <- factor(z)
y ~ x2+z2
--
O__ ---- Peter Dalgaard Blegdamsvej 3
c/ /'_ --- Dept. of Biostatistics 2200 Cph. N
(*) \(*) -- University of Copenhagen Denmark Ph: (+45) 35327918
~~~~~~~~~~ - (p.dalgaard at biostat.ku.dk) FAX: (+45) 35327907
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