In this article by Terry Therneau 2016, to address time-varying effect,
Testing proportional hazard assumption:
vfit <- coxph(Surv(time,status) ~ trt + prior + karno, data = veteran) zp <- cox.zph (vfit, transform=function(time) log(time+20) zp
Extended Cox model addressing time-varying effect:
vfit3 <- coxph (Surv(time,status) ~ trt + prior + karno + tt(karno), data = veteran, tt = function (x,t, ...) x*log(t+20))
I am concerned with usage of log(t+20) that author used in both analyses (testing assumption and addressing time-varying effect). why using specifically log(t+20) rather than just log(t) which is commonly used by some people (I think)?
*Something I would like to add, not directly related to my post question:
I strongly suggest someone with rep above 300 add a new tag "time-varying-coefficients" (only "time-varying-covariates" exist, but the two are different and very important to distinguish them). With appropriate 'tag' make post efficiently be searched. Thanks in advance.*