Unobserved Heterogeneity

Unobserved heterogeneity is outside the scope of the note's perturbation derivations. The note does not incorporate formal relationships for unobserved heterogeneity into its perturbation derivations. Frailty differences matter because saved people may have lower remaining life expectancy than people who did not need rescue. Extending the framework to frailty may reveal cases where lifespan-inequality measures disagree under selection processes.