Hi, I am trying to internally validate the Cox proportional hazard model. I have split data (70% on which I have constructed the model and 30% on which I am trying to validate it). As for CPH model, we use Harrell's C (calculated using estat concordance) and it only runs on the data on which the model was constructed (70%) not on the data (30%) I am trying to validate. Is there any way I can use estat concordance in my validation sample to see how the model is performing? Any help will be appreciated. Thanks, Zia