@conference {liu2012exploring,
	title = {Exploring and predicting search task difficulty},
	booktitle = {Proceedings of the 21st ACM international conference on Information and knowledge management},
	year = {2012},
	pages = {1313{\textendash}1322},
	author = {Jingjing Liu and Chang Liu and Cole, Michael and Belkin, Nicholas J. and Zhang, Xiangmin}
}
