The role of the frailty in survival studies

The focus is mainly, but, not solely, on longitudinal randomised controlled clinical trials. The paper aims to delineate the role of frailty in the modern analysis of such trials and also in longitudinal survival studies. Our approach exploits recent developments in statistical modelling and in estimation methods, for example, in non-PH survival modelling, covariance modelling and in h-likelihood inference. We illustrate our approach and findings with examples from the literature.