@inproceedings{questiondifficultyahowtoestimatewithout,
  title = {Question Difficulty -- How to Estimate Without Norming, How to Use for Automated Grading},
  author = {Ulrike Pad{\'o}},
  year = {2017},
  booktitle = {WS 2017 9},
  url = {https://aclanthology.org/W17-5001},
}