[R] Contr.sum and coefficient tests

[François Collin]

Dear all,

I would like to run a linear model which includes two factors:
- The first one has two levels, including a reference level. Thus I have to use the treatment contrast (contr.treatment, reference level effect = 0, then the intercept).
- The second is a 6-level factor without reference contrast nor order. So, I would like to use sum contrat: sum of the effects = 0.

The problem arises when it comes to the coefficient test. I understand it is not relevant to test the reference level for the first factor as the reference level is set to 0. However, using sum contrast for the second factor, I would have expected the test of each level to be included in the classical summary print of the lm function result but it is not. And here is my problem, how can I have every coefficients tested and printed in the summary output when my factor is studied from this sum.contrast standpoint?


Here is an example from a previous thread:
(http://comments.gmane.org/gmane.comp.lang.r.general/258886)

==============================
> x <- as.factor(c(1,1,1,2,2,2,2,2,2,3,3,3,3,3,3,3,3,3,3,3))
> y <- c(1.1,1.15,1.2,1.1,1.1,1.1,1.2,1.2,1.2,2.1,2.2,2.3,2.4,2.5,
> +          2.6,2.7,2.8,2.9,3,3.1)
> test <- data.frame(x,y)
> reg1 <- lm(y~C(x,contr.sum),data=test)
> summary(reg1)

Coefficients:
                 Estimate Std. Error t value Pr(>|t|)   
(Intercept)       1.63333    0.06577  24.834 8.48e-15 ***
C(x, contr.sum)1 -0.48333    0.10792  -4.479  0.00033 ***
C(x, contr.sum)2 -0.48333    0.08936  -5.409 4.70e-05 ***
==============================

How can I include the third factor level in this table?

I fill the answer provided in this previous thread is relevant for contr.treatment but not for contr.sum. Am I right? Or can you explain me?

Many thanks,
Fanch



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