CEFR wordlists on based on data from the following sources:
- Cambridge Learner Corpus (CLC). This is a collection of several hundred thousand examination scripts written by learners from 203 different countries.
- Level-specific examination vocabulary lists and classroom materials.
- Cambridge English Corpus, a multi-billion word corpus of spoken and written English.
- Additional sources for the C levels wordlists include reference lists relevant to academic English.
