These are the studies on this topic that I have presented at various conferences:
Kim, S. (2019, March). Assessing grammatical features in second language writing: A corpus-based analysis for rating scale validation. Paper to be presented at the American Association for Applied Linguistics (AAAL) conference, Atlanta, Georgia, USA.
Kim, S. (2018, September). Predicting CEFR proficiency level using grammatical criterial features. Paper presented at the American Association for Corpus Linguistics (AACL) conference, Atlanta, Georgia, USA.
Kim, S., Reed, D. J., Ohlrogge, A., Kim, H., Van Gorp, K., Isbell, D. R. (2018, March). Investigating the textual complexity of essay benchmarks at different levels on a scale within the CEFR B2 range. Work-in-progress presented at the Language Assessment Research Conference (LARC), Ames, Iowa, USA.
Kim, S. (2017, March). Examining the grammatical criterial features for CEFR levels in a learner corpus of written English. Paper presented at the American Association for Applied Linguistics (AAAL) conference, Portland, Oregon, USA.