-- begin included message -- Hi, but why we do the difference : ltemp <- 2 * diff(tfit $loglik[1:2]) ?? Where I can find information about Integrate Likelihooh and null like lihood?? --- end inclusion --- 1. Basic statistical fact: 2 * difference in loglik between two nested models = distributed as a chi-square distribution. For coxme loglik[1] = likelihood from a null model (all coefs =0). 2. In a random effects model the random terms are integrated out of the likelihood. This is a large topic and it's not easy to give a simple reference other than 'read one of the class textbooks on mixed effects models'.