Know What You Don’t Know: Unanswerable Questions for SQuAD Pranav Rajpurkar author Robin Jia author Percy Liang author 2018-07 text Proceedings of the 56th Annual Meeting of the Association for Computational Linguistics (Volume 2: Short Papers) Iryna Gurevych editor Yusuke Miyao editor Association for Computational Linguistics Melbourne, Australia conference publication rajpurkar-etal-2018-know 10.18653/v1/P18-2124 https://aclanthology.org/P18-2124/ 2018-07 784 789