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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)
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When you do not have stratification in the Cox PH model, $D_{xy}$ is simple and treats time as a continuous variable, so you do not get separate $D_{xy}$ for different time cutoffs, unlike for calibration curves. $D_{xy}$ is the Somers' rank correlation between predicted log relative hazard and observed survival time.

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