[R] How to calculate confidence interval of C statistic by rcorr.cens

[khosoda at med.kobe-u.ac.jp]

Hi,

I'm trying to calculate 95% confidence interval of C statistic of
logistic regression model using rcorr.cens in rms package. I wrote a
brief function for this purpose as the followings;

CstatisticCI <- function(x)   # x is object of rcorr.cens.
  {
    se <- x["S.D."]/sqrt(x["n"])
    Low95 <- x["C Index"] - 1.96*se
    Upper95 <- x["C Index"] + 1.96*se
    cbind(x["C Index"], Low95, Upper95)
  }

Then,

> MyModel.lrm.rcorr <- rcorr.cens(x=predict(MyModel.lrm), S=df$outcome)
> MyModel.lrm.rcorr
       C Index            Dxy           S.D.              n
missing     uncensored
     0.8222785      0.6445570      0.1047916    104.0000000
0.0000000    104.0000000
Relevant Pairs     Concordant      Uncertain
  3950.0000000   3248.0000000      0.0000000

> CstatisticCI(x5factor_final.lrm.pen.rcorr)
                      Low95   Upper95
C Index 0.8222785 0.8021382 0.8424188

I'm not sure what "S.D." in object of rcorr.cens means. Is this standard
deviation of "C Index" or standard deviation of "Dxy"?
I thought it is standard deviation of "C Index". Therefore, I wrote the
code above. Am I right?

I would appreciate any help in advance.

--
Kohkichi Hosoda M.D.

    Department of Neurosurgery,
    Kobe University Graduate School of Medicine,