[R] model formula

[Peter Dalgaard]

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