[R] ANOVA with both discreet and continuous variable

[array chip]

Hi all,

I have dataset with 2 independent variable, one (x1)
is continuous, the other (x2) is a categorical
variable with 2 levels. The dependent variable (y) is
continuous. When I run linear regression y~x1*x2, I
found that the p value for the continuous independent
variable x1 changes when different contrasts was used
(helmert vs. treatment), while the p values for the
categorical x2 and interaction are independent of the
contrasts used. Can anyone explain why? I guess the p
value for x1 is testing different hypothesis under
different contrasts? If the interaction is NOT
significant, what contrast should I use to test the
hypothesis that x1 is not significantly related with
y?


x1<-rnorm(50,9,2)
x2<-as.factor(as.numeric(runif(50)<0.35))
y<-rnorm(50,30,5)

options(contrasts=c('contr.treatment','contr.poly'))
summary(lm(y~x1*x2))

options(contrasts=c('contr.helmert','contr.poly'))
summary(lm(y~x1*x2))