@inproceedings{miningwordsinthemindsofsecondlanguage,
  title = {Mining Words in the Minds of Second Language Learners: Learner-Specific Word Difficulty},
  author = {Yo Ehara and Issei Sato and Hidekazu Oiwa and Hiroshi Nakagawa},
  year = {2012},
  booktitle = {COLING 2012 12},
  url = {https://aclanthology.org/C12-1049},
}