@InProceedings{liu-EtAl:2013:EMNLP,
  author    = {Liu, Jing  and  Wang, Quan  and  Lin, Chin-Yew  and  Hon, Hsiao-Wuen},
  title     = {Question Difficulty Estimation in Community Question Answering Services},
  booktitle = {Proceedings of the 2013 Conference on Empirical Methods in Natural Language Processing},
  month     = {October},
  year      = {2013},
  address   = {Seattle, Washington, USA},
  publisher = {Association for Computational Linguistics},
  pages     = {85--90},
  url       = {http://www.aclweb.org/anthology/D13-1009}
}