@article{Heckman2006,
author = {Heckman, J. and Urzua, S. and Vytlacil, E.},
title= {Understanding instrumental variables in models with essential heterogeneity},
year = {2006},
doi = {10.1162/rest.88.3.389},
journal = {Review of Economics and Statistics},
volume = {88},
number = {3},
pages = {389--432},
}