[R] coxph - weights- robust SE

Hi, I am using coxph with weights to represent sampling fraction of subjects. Our simulation results show that the robust SE of beta systematically under-estimate the empirical SD of beta. Does anyone know how the robust SE are estimated in coxph using weights? Is there any analytical formula

[R] problem with coxph

Hi everyone, I encountered a problem using the coxph function for the conditional logistic regression. I am trying to do some simulations and I really don’t understand a mistake which happened maybe only 1 time among more than 1,000 simulations. What appeared on the screen is the following: