[EMAIL PROTECTED] (Christian Lederer) writes: > Hi, > > perhaps this is a stupid question, but i need some help about > Helmert contrasts in the Cox model. > > > I have a survival data frame with an unordered factor `group' > with levels 0 ... 5. > Calculating the Cox model with Helmert contrasts, i expected that > the first coefficient would be the same as if i had used treatment > contrasts, but this is not true. > I this a error in reasoning, or is it possible, that both contrasts use > a different ordering of the levels? > I am confused about the fact, that coefficients for different levels > are calculated, depending on weather i include level 0 or not: > > If level 0 is included, with both contrasts coxph calculates > coefficients for group 1 ... group 5. > But if i exclude level 0, coxph calculates coeffficients for > group 2 ... 5 with treatment and coefficients for group 1 ... 4 using > helmert: ..... > A am absolutely confused; any help is welcome.
You are not the first... Probably the best way of becoming less confused is to look at the contrast matrices: > contr.treatment(5) 2 3 4 5 1 0 0 0 0 2 1 0 0 0 3 0 1 0 0 4 0 0 1 0 5 0 0 0 1 > contr.helmert(5) [,1] [,2] [,3] [,4] 1 -1 -1 -1 -1 2 1 -1 -1 -1 3 0 2 -1 -1 4 0 0 3 -1 5 0 0 0 4 These give the coefficient matrices for mapping from regression coefficients to individual group levels (determined up to an additive constant of course). Staring long enough at these you'll realize that the first treatment contrast is the difference between gr1 and gr2 but the first Helmert contr. is *half* that difference, etc. -- O__ ---- Peter Dalgaard Blegdamsvej 3 c/ /'_ --- Dept. of Biostatistics 2200 Cph. N (*) \(*) -- University of Copenhagen Denmark Ph: (+45) 35327918 ~~~~~~~~~~ - ([EMAIL PROTECTED]) FAX: (+45) 35327907 ______________________________________________ [EMAIL PROTECTED] mailing list https://www.stat.math.ethz.ch/mailman/listinfo/r-help PLEASE do read the posting guide! http://www.R-project.org/posting-guide.html
