@article{Heckman1984,
author = {Heckman, J.J. and Singer, B.S.},
title= {A Method for Minimizing the Impact of Distributional Assumptions in Econometric Models for Duration Data},
year = {1984},
doi = {10.2307/1911491},
journal = {Econometrica},
volume = {52},
number = {2},
pages = {271--320},
}