@article{Bock_1981,
title={Marginal Maximum Likelihood Estimation of Item Parameters: Application of an EM Algorithm},
volume={46},
ISSN={1860-0980},
url={http://dx.doi.org/10.1007/bf02293801},
DOI={10.1007/bf02293801},
number={4},
journal={Psychometrika},
publisher={Cambridge University Press (CUP)},
author={Bock,
R. Darrell and Aitkin,
Murray},
year={1981},
month=dec,
pages={443–459} }