Sentence difficulty evaluation for a learner's dictionary

In order to choose the best example sentences for Chinese language learners, we have compared how well different methods of estimating difficulty (reading time, translation time, direct rating) could be approximated from a sentence's superficial features. We have found that direct rating of difficulty by a user is the most promising, and that character-based features allow for better evaluation than word-based ones, all of which bodes well for user-tailored learner's dictionaries.