Dear Dominik, I'm not familiar with contr.SAS (where does it come from?), but from the output I'd guess that it simply sets a different baseline category for the contrasts -- the last category (in your, case, coded 1 apparently) rather than the first (i.e., coded 0), which is what contr.treatment does. Notice that you have the same estimate and standard error for the interaction in both cases, which is consistent with my guess. To verify the codings try contrasts(factor(PRO)) in each case. In general, you should interpret parameter estimates in conformity with the coding employed. When using 0/1 coding in a model with interactions, you shouldn't think of the the coefficients for factor(PRO) and factor(PRE) as "main effects." None of this is specific to the Cox regression model. I hope that this helps, John
At 10:36 AM 11/7/2002 +0100, Grathwohl,Dominik,LAUSANNE,NRC/NT wrote:
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?
----------------------------------------------------- John Fox Department of Sociology McMaster University Hamilton, Ontario, Canada L8S 4M4 email: jfox at mcmaster.ca phone: 905-525-9140x23604 web: www.socsci.mcmaster.ca/jfox ----------------------------------------------------- -.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.- r-help mailing list -- Read http://www.ci.tuwien.ac.at/~hornik/R/R-FAQ.html Send "info", "help", or "[un]subscribe" (in the "body", not the subject !) To: r-help-request at stat.math.ethz.ch _._._._._._._._._._._._._._._._._._._._._._._._._._._._._._._._._._._._._._._._