One of my colleagues has written a technical report on how to do this, but I
have not yet implemented it in the survival package.
Terry Therneau
http://mayoresearch.mayo.edu/mayo/research/biostat/techreports.cfm
#80
Concordance for Survival Time Data: Fixed and Time-Dependent Covariates and
Possible Ties in Predictor and Time
Concordance, or synonymously the C-statistic, is a valuable measure of model
discrimination in analyses involving survival time data. This report provides a
definition of concordance in the case of survival data, allowing for
time-dependent covariates with the counting process data representation and
accounting for ties in the covariates and times.
Walter K Kremers and The William J. von Liebig Transplant Center [April 2007]
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