Estimates are derived using Cox proportional-hazards models (Cox, 1972), a regression model commonly used in medical research for investigating the association between the survival time of patients and one or more predictor variables. Survival models relate the time that passes, before some event occurs, to one or more covariates that may be associated with that quantity of time. In a proportional hazards model, the unique effect of a unit increase in a covariate is multiplicative with respect to the hazard rate.