Frailty models are mixed-effects models for survival data. The terminology
originates from medical science, where the word frailty denotes a general
susceptibility to ill health. As a random effect, a frailty term can be specified
for an individual, but can also be cluster-specific in which case individuals
within clusters share the same frailty. Applications of frailty models are not
limited to medical science but can also be found in other disciplines where
survival analysis is undertaken.