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

[Eleni Rapsomaniki]

Dear R-users,

One can use the rcorr.cens function in Design to compute the C index when only the stop time is indicated (I think implicitely start=0 in that case). When the start and stop times are used in a Surv object, e.g. 

library(Design)
S=with(heart, Surv(start,stop,event))

the object returned is no longer a single number vector, so none of the following ways to compare a model fit to S makes sense:

rcorr.cens(-cph(S~ age+transplant+surgery, heart)$linear.predictors, with(heart, Surv(start,stop,event)))
#       C Index            Dxy           S.D.              n        missing 
#      4.95e-01      -9.66e-03       7.15e-02       1.72e+02       0.00e+00 
#this seems bad because it compares the fit to a different Surv than the one used to fit it (S, above)

rcorr.cens(-cph(S~ age+transplant+surgery, heart)$linear.predictors, with(heart, Surv(stop-start,event)))
#       C Index            Dxy           S.D.              n        missing 
#      6.02e-01       2.03e-01       7.54e-02       1.72e+02       0.00e+00 

Is there some way I can compute the correlation for this type of survival time specification in R? If not, does anybody have some tips on how to go about computing it myself?

Many Thanks

Eleni Rapsomaniki
Research Associate
Strangeways Research Laboratory
Department of Public Health and Primary Care
University of Cambridge