The Lexile Framework for Reading was commissioned by the U.S. government agency - the National Institute of Child Health and Human Development, the American Metametircs Education Company, and spent more than 20 years working with children's language development professors and tens of thousands of students to develop a set of A grading mechanism that measures a reader's reading comprehension and difficulty of a book.

Lexile gives each text a Lexile value by analyzing semantic difficulty, syntactic difficulty, structure, and vocabulary, ranging from BR (Beginning Reader) to 2300L. The higher the value, the higher the difficulty of the text. At the same time, readers can also find suitable reading materials through this evaluation standard. Lexile reading grading has developed into the most credible reading grading system in the United States. Major examination institutions, such as IELTS, TOEFL Junior, and TOEFL Primary, also provide comparison values between test scores and Lexile scores.