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: [EMAIL PROTECTED] web: http://www.socsci.mcmaster.ca/jfox
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