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Paweł Budzianowski, Tsung-Hsien Wen, Bo-Hsiang Tseng, Iñigo Casanueva, Stefan Ultes, Osman Ramadan, Milica Gašić. Proceedings of the 2018 Conference on Empirical Methods in Natural Language Processing. 2018.
Budzianowski et al. (Mon,) studied this question.