What makes a query difficult?

@inproceedings{carmel-2006-makes, author = {David Carmel and Elad Yom-Tov and Adam Darlow and Dan Pelleg}, editor = {Efthimis N. Efthimiadis and Susan T. Dumais and David Hawking and Kalervo J\"{a}rvelin}, title = {{What makes a query difficult?}}, booktitle = {{SIGIR 2006: Proceedings of the 29th Annual International ACM SIGIR Conference on Research and Development in Information Retrieval, Seattle, Washington, USA, August 6-11, 2006}}, pages = {390--397}, publisher = {ACM}, year = {2006}, url = {https://doi.org/10.1145/1148170.1148238}, doi = {https://doi.org/10.1145/1148170.1148238} }