[R] C-index for models fitted using start, stop in Surv?

[Terry Therneau]

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]