@article{Ridder2003,
author = {Ridder, G. and Woutersen, T.M.},
title= {The Singularity of the Information Matrix of the Mixed Proportional Hazard Model},
year = {2003},
doi = {10.1111/1468-0262.00460},
journal = {Econometrica},
volume = {71},
number = {5},
pages = {1579--1589},
}