Starting from 1990, Corpus of Contemporary American English (COCA) picks 20 million words of English text and speech each year, and now contains more than 520 million words. Based on COCA, this series of The Graded Wordbook of 52,000 Words are grouped in 1,000 words and lead ultimately to 52,000 most frequently-used English words.

These words can be divided into 6 larger groups:

4,000 Elementary Words---Corresponding to the vocabulary size of elementary school students

4,000 Intermediate Words---Corresponding to the vocabulary size of middle school students

5,000 Upper-intermediate Words---Corresponding to the vocabulary size of high school students, TOEFL, IELTS, SAT, ACT

5,000 Advanced Words---Corresponding to the vocabulary size of college students, SAT, ACT, GRE, GMAT

12,000 Superior Words---Corresponding to the vocabulary size of graduate students, GRE, GMAT

22,000 Ultimate Words---Corresponding to the vocabulary size of most extensive readers