@article{Therneau2014,
author = {Therneau, T.M. and Crowson, C.},
title= {Using time dependent covariates and time dependent coefficients in the Cox model},
year = {2014},
URL = {http://cran.r-project.org/web/packages/survival/vignettes/timedep.pdf
%[21 March 2014},
}