https://en.wikipedia.org/wiki/Heckman_correction
"Conceptually, this is achieved by explicitly modelling the individual sampling probability of each observation (the so-called selection equation) together with the conditional expectation of the dependent variable (the so-called outcome equation)"
The Heckman used Mills ratio to model right-censored data as 'selection bias'.
https://papers.ssrn.com/sol3/papers.cfm?abstract_id=3659766
`selection bias' using the inverse Mills ratio neglected by epidemiologists.
https://en.wikipedia.org/wiki/Mills_ratio
Mills ratio is the Survival function / pdf,
The Gompertz function is u = - dS/dt * 1/S, so, HQ thinks Mills ratio is the inversion of the Gompertz mortality rate.
related to
https://en.wikipedia.org/wiki/Heckman_correction
