Odp: Factor and Lm functions
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
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