@article{Nielsen1992,
author = {Nielsen, G.G. and Gill, R.D. and Andersen, P.K. and Sørensen, T.A.I.},
title= {A counting process approach to maximum likelihood estimation in frailty models},
year = {1992},
journal = {Scandinavian Journal of Statistics},
volume = {19},
pages = {25--43},
}