I am using RMS package of R to validate cox regression model with bootstrap. Please see the R code below. I am predicting 1, 2, 3, 4, 5 years survival. Is the optimism corrected discrimination index Dxy a single index for the model, regardless of which year I am predicting, or does it have a different value for predicting each year survival? When “i” is specified to be 1, 2, 3, 4, 5 (years), the optimism corrected Dxy changes. Is this because of the change in the year? Or is it just due to the different random samples generated in each run?
f = cph(s1 ~ GRADE_f_imp + METAST_f_imp + largests_2b_imp + lymphnod_f_imp + LNPOS_f_imp + genderma_f_imp, data=d_OS, x=T, y=T, surv=T, time.inc=i*12) validate (f, B=200, u=i*12)