In order to Lexile a book or article, text is split into 125-word slices. Each slice is compared to the nearly 600-million word Lexile corpus – taken from a variety of sources and genres – and words in each sentence are counted. The sentence length and difficulty of the vocabulary is examined throughout the book. These calculations are put into the Lexile equation. Then, each slice’s resulting Lexile measure is applied to the Rasch psychometric model to determine the Lexile measure for the entire text.
Lexile measures do NOT measure age-appropriateness, the book quality, the book's theme or other such characteristics of the book. For example, The Grapes of Wrath is a fairly simple book to read but may have a theme that is inappropriate for a certain age group. The Diary of a Wimpy Kid may have more complex words and phrases and therefore has a higher Lexile measure.
There is a video that may also help explain this: http://www.youtube.com/watch?v=fr0jQzrDafw. You can check out our other
videos here: http://www.lexile.com/about-lexile/lexile-video/