[R] Odp: Factor and Lm functions

[Petr PIKAL]

Hi


r-help-bounces at r-project.org napsal dne 04.02.2009 12:33:57:

> 
> Hei,
> I have a formula for a model as follows
> lm(TS~log(BodyWt)+log(BodyWt):factor(D). I do not use R for programming
> hence I dont understand what is the second covariate in the model
> "log(BodyWt):factor(D)".
> Where BodyWt = body weight and D = danger index (either 1 or 2). I want 
to
> run the same model in other program. Can anyone explain me what is doing 
the
> : operator and the factor() function.

Factor changes D from numeric (1,2) to factor with levels 1 and 2. As a 
result from lm with such specified model you shall get one intercept and 2 
slopes one for level 1 and second for level 2 of D

see

x<-1:20
A<-sample(1:2, 20, replace=T)
y<-10+x*5*(A==1)+x*12*(A==2)+rnorm(20)
fit<-lm(y~x+x:factor(A))
plot(x,y)
summary(fit)
lines(x, 9.38+x*5.07268)
lines(x, 9.38+x*5.07268+x*6.98)

Regards
Petr


> 
> Thanks
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