@article{Heckman1998,
author = {Heckman, J.J. and Ichimura, H. and Smith, J. and Todd, P.},
title= {Characterizing selection bias using experimental data},
year = {1998},
doi = {10.3386/w6699},
journal = {Econometrica},
volume = {66},
number = {5},
pages = {1017--1098},
}