Here is an article from Oxford University Press ELT Blog.
When we create assessments, why is it important to make sure vocabulary is at a particular level?
When we create tests, we have to know that learners have sufficient vocabulary to engage with the reading and listening materials. If the level is too high, the texts will not be accessible and the test will be too difficult. Alternatively, if the vocabulary is too basic, the test may be too easy. Consequently, when we develop reading and listening materials for Oxford University Press, we have to make sure that the materials are challenging for test-takers, but not too difficult. To do this, we’ve created special word lists.
The Oxford 5000 is a list of the most important words for learners to acquire as they progress from A1-C2 of the CEFR. The list includes basic function words, common verbs, everyday nouns and adjectives. Corpus analysis shows that about 80% of almost any English text is made up of the 2,000 most frequent words. Therefore, knowledge of the 3,000 most frequent words in English gives learners a very high coverage of the words they are likely to see and hear.