Thomas L, Reyes EM. Tutorial: Survival Estimation for Cox Regression Models with Time-Varying Coefficients Using SAS and R. J. Stat. Soft. [Internet]. 2014 Oct. 24 [cited 2024 Nov. 21];61(1):1-23. Available from: https://www.jstatsoft.org/index.php/jss/article/view/v061c01