Diffusion and Lévy process models for dynamic frailty

In Chapter 10, we demonstrated how the concept of dynamically developing individual risk could be used to produce frailty-like effects on the population level. As soon as the risk process of an individual hits a barrier, that individual has had an event, leaving behind only those with a risk process not yet at the level of the barrier. The barrier hitting models can be seen as a way of relaxing the assumption of time-constant frailty found in the usual frailty models (Chapter 6). Another way of relaxing the constancy assumption is to allow dynamic covariates to capture a part of the variation experienced by an individual, as demonstrated in Chapter 8.