@article{Hess1995,
author = {Hess, K.R.},
title= {Graphical methods for assessing violations of the proportional hazards assumption in Cox regression},
year = {1995},
doi = {10.1002/sim.4780141510},
journal = {Statistics in Medicine},
volume = {14},
number = {15},
pages = {1707--1723},
}