@inbook{Heckman1976,
author = {Heckman, J.J.},
title= {The common structure of statistical models of truncation, sample selection and limited dependent variables and a simple estimator for such models},
year = {1976},
booktitle = {Annals of Economic and Social Measurement, Volume 5, number 4},
publisher = {NBER},
pages = {475--92},
}