tokens: In addition , they have used features such as the number of words grammatically related to the target word , psycholinguistic features from the MRC database , CEFR ( Common European Framework of Reference for Languages ) levels extracted from the Cambridge Advanced Learner Dictionary ( CALD ) , and Google N - gram word frequencies using the Datamuse API The MCR features include word familiarity rating , number of phonemes , thorndike - lorge written frequency , imageability rating , concreteness rating , number of categories , samples , and written frequencies , and age of acquisition .