I wonder if there is any research on how many vocabulary is required for each TOPIK level. It should be something like the following:
- TOPIK 1: 1000 words
- TOPIK 2: 2500 words
- TOPIK 3...
And there should be an objective threshold on each level, such as the number of words to cover X% of the entire texts in the past Y years of TOPIK tests.
Is there any such research effort?