[R] model formula
John Fox
jfox at mcmaster.ca
Tue Nov 18 19:57:28 CET 2003
Dear Bill,
I believe that lm(y ~ z + I((z == 1)*x)) will give you what you want.
I hope that this helps,
John
At 08:40 AM 11/18/2003 -0500, you wrote:
>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)
>
>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?
____________________________
John Fox
Department of Sociology
McMaster University
email: jfox at mcmaster.ca
web: http://www.socsci.mcmaster.ca/jfox
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