[R] rcorrp.cens and U statistics

[pietro bulian]

I have two alternative Cox models with C-statistics 0.72 and 0.78. My question is if 0.78 is significantly greater than 0.72. I'm using rcorrp.cens. I cannot find the U statistics in the output of the function. This is the output of the help example:

> x1 <- rnorm(400)
> x2 <- x1 + rnorm(400)
> d.time <- rexp(400) + (x1 - min(x1))
> cens   <- runif(400,.5,2)
> death  <- d.time <= cens
> d.time <- pmin(d.time, cens)
> rcorrp.cens(x1, x2, Surv(d.time, death))
               Dxy               S.D. x1 more concordant x2 more concordant                  n            missing         uncensored     Relevant Pairs 
     -8.902077e-03       2.649712e-01       4.955490e-01       5.044510e-01       4.000000e+02       0.000000e+00       4.000000e+00       1.348000e+03 
         Uncertain               C X1               C X2             Dxy X1             Dxy X2 
      1.582520e+05       9.955490e-01       9.762611e-01       9.910979e-01       9.525223e-01 

Should I manually calculate the U statistics interpreting the fraction "x1 more concordant" and "x2 more concordant" of the "Relevant Pairs" as Uxy and Uyx? 

many thanks for any help or suggestion

Pietro Bulian