[R] Preferable contrasts?

[Grathwohl,Dominik,LAUSANNE,NRC/NT]

Dear all,

I'm working with Cox-regression, because data could be censored. 
But in this particular case not. 
Now I have a simple example: PRO and PRE are (0,1) coded.
The response is not normal distributed. 
We are interested in a model which could describe interaction.
But my results are depending strongly in the choose of the contrast option.
It is clear that there is some dependence in the contrasts, but in this
simple case 
I could get the vice versa effect.
My R output:

> options(contrasts = c(unordered = "contr.treatment", ordered =
"contr.poly"))
> summary(coxph(Surv(ILOG, alive) ~ factor(PRO)*factor(PRE)))
...
                             coef exp(coef) se(coef)      z     p
factor(PRO)1               0.6576     1.930    0.302  2.177 0.029
factor(PRE)1               0.0681     1.070    0.304  0.224 0.820
factor(PRO)1:factor(PRE)1 -0.7703     0.463    0.431 -1.789 0.074
...
> options(contrasts = c(unordered = "contr.SAS", ordered = "contr.poly"))
> summary(coxph(Surv(ILOG, alive) ~ factor(PRO)*factor(PRE)))
...
                            coef exp(coef) se(coef)      z     p
factor(PRO)0               0.113     1.119    0.304  0.370 0.710
factor(PRE)0               0.702     2.018    0.299  2.350 0.019
factor(PRO)0:factor(PRE)0 -0.770     0.463    0.431 -1.789 0.074
...

What would the experts recommend?

Kind regards,

Dominik

Dominik Grathwohl 
Biostatistician 
Nestlé Research Center 
PO Box 44, CH-1000 Lausanne 26 
Phone: + 41 21 785 8034 
Fax: + 41 21 785 8556 
e-mail: dominik.grathwohl at rdls.nestle.com 


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