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Article 1704530

What is the ATOS Readability Formula?

Product: Renaissance Accelerated Reader

Platform: Macintosh, Windows

Last Updated: 10/7/2019



What is the ATOS Readability Formula?


An ATOS level on a book indicates how difficult the text is to read. Accelerated Reader uses the ATOS Readability Formula to help teachers guide student reading practice by assigning a readability level (book level) to books.  The ATOS Readability Formula is one of the most widely used measures for judging the difficult level of text. The product of extensive research, the formula employs several unique features:

  • Takes into account characteristics that most heavily influence readability (average sentence length, average word length, and word difficulty level)
  • Utilizes a graded vocabulary list number more than 100,000 words
  • Bases analyses on worlds largest database of words from actual books (more than 170,000 books comprising nearly 2.5 billion words)

The ATOS Readability Formula measures only the readability of the text.  It does not (nor do any other readability formulas) take into account literary merit, sophistication of ideas, developmental appropriateness, or maturity of theme or language.  This is why it's important to know and use three measurements when helping your students choose books: readability level (the difficulty of a book's text), reading level (the student's tested reading level), and interest level (the content level of a book).

An ATOS book level is often a good indication that a student can read a book but may not mean that he or she should read the book. The interest level can help you decide whether the student should read the book.


Keywords: ATOS, formula, reading formula, readability formula, reading levels, reading level, book levels, book level, Accelerated Reader, 1704530, ZKBInstallation

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