Wikipedia
The Lexile framework for reading is an educational tool that uses a measure called a Lexile to match readers with books, articles and other leveled reading resources. Readers and books are assigned a score on the Lexile scale, in which lower scores reflect easier readability for books and lower reading ability for readers.
The Lexile framework uses quantitative methods, based on individual words and sentence lengths, rather than qualitative analysis of content to produce scores. Accordingly, the scores for texts do not reflect factors such as multiple levels of meaning or maturity of themes. Hence, in the United States, the Common Core State Standards recommend the use of alternative, qualitative, methods for selecting books for students at grade 6 and over.
In the US, Lexile measures are reported from reading programs and assessments annually. Thus, about half of U.S. students in grades 3rd through 12th receive a Lexile measure each year. In addition to being used in schools in all 50 states, Lexile measures are also used outside of the United States.