@inproceedings{predictingthedifficultyoflanguage,
  title = {Predicting the Difficulty of Language Proficiency Tests},
  author = {Lisa Beinborn and Torsten Zesch and Iryna Gurevych},
  year = {2014},
  booktitle = {TACL 2014 1},
  url = {https://aclanthology.org/Q14-1040},
}