On Tue, 2007-11-20 at 16:39 +0100, Irene Mantzouni wrote:
> Hi all!
>
> Is it possible to model a multiple regression in which the response becomes
> zero when one of the two covariates is zero?
> lm(y~ x1+x2) and y=0 if x1=0.
> However, when x1=0, y=x2+1(intercept).
> Does this mean I cannot have a second covariate and intercept or should I
> eliminate only the intercept?
>
> thank you!
Well,
If y=x2+1 as same y-1=x2
So you can use lm(y-1~x2-1)
Look this example
set.seed(123)
y<-rnorm(100,sd=2)
x2<-rpois(100,25)
lm(y-1~x2-1)
Call:
lm(formula = y - 1 ~ x2 - 1)
Coefficients:
x2
-0.03041
Your regression is y-1=-0.03041*x2 or y=1-0.03041*x2
--
Bernardo Rangel Tura, M.D,MPH,Ph.D
National Institute of Cardiology
Brazil
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