@article{Hansen2004,
author = {Hansen, K.T. and Heckman, J.J.  and Mullen, K.},
title= {The effect of schooling and ability on achievement test scores},
year = {2004},
doi = {10.1016/j.jeconom.2003.10.011},
journal = {Journal of Econometrics},
volume = {121},
number = {1–2},
pages = {39--98},
}