[R] cox models

[bertrand]

Hello,
i'm a french student of medical oncology and i'm working on breast 
cancer. I have a variable with the histologic type of tumor wich is 
between 1 and 5. I use as.factor function to make some variable with 
level between 1 and 5. When i put it in the cox model i have only the 
level between 2 and 5. The level 1 doesn't appear. I think i have to 
change the number of level but i don't really know. Please can you help 
me?


Class
Levels: 1 2 3 4 5
coxph(formula = Surv(delai.etat, etat) ~ class, data = igr1)


          coef exp(coef) se(coef)     z       p
class2 -0.093     0.911    0.245 -0.38 7.0e-01
class3 -0.749     0.473    0.286 -2.62 8.9e-03
class4 -0.600     0.549    0.343 -1.75 8.0e-02
class5 -0.874     0.417    0.197 -4.44 8.9e-06

Likelihood ratio test=24.9  on 4 df, p=5.28e-05  n=740 (1 observations 
deleted due to missing)



Thanks for your help


Bertrand