What are AR book levels… and what are they not?

By Cecelia Powell, Managing Editor

Accelerated Reader helps educators to guide students to books that will provide an appropriate level of challenge. Students using AR are regularly tested to determine their “Zone of Proximal Development”, which corresponds to the range of text complexity within which they ought to be reading in order to make the greatest progress with their reading skills. Too easy, and a book won’t challenge a student. Too difficult, and a book will cause frustration.

In order to help educators match books to students, every book with an Accelerated Reader quiz has been assigned a book level. The book level is a measure of the complexity of the text. ATOS, the readability formula used to assess text complexity for AR, is applied consistently to every one of the 26,000+ books we have quizzed.

The ATOS formula used in Accelerated Reader analyses the full text of a book, not merely short passages. The book level is determined by the characteristics that most heavily influence readability: average sentence length, average word length and word difficulty level. The word difficulty levels have been determined for over 100,000 words, and are based on analyses of the world’s largest database of words from actual books (more than 170,000 books comprising of nearly 2.5 billion words).

What book levels are and what they are not

There are two important implications to this, which cannot be stressed clearly enough.

ATOS book levels are consistent, reliable and objective measures of text complexity. The formula takes into consideration the most important factors in determining the complexity of text and uses the largest data sample of its kind in the world to determine the results. ATOS is reliable in what it measures and is an extremely useful guide for librarians, parents and teachers.

However, ATOS book levels are not the only measures of the suitability of a given book for a particular student. No scientific formula can take into consideration the maturity of the themes addressed or the sophistication of the literary devices employed by the author of a book. Discretion and professional judgment are vital ingredients to successful classroom practice when guiding students to appropriate books.

Using book levels in the classroom

The role of librarians

Accelerated Reader was never intended to replace the role of the educator. While self-selection of books by students in a given range of book levels is an important component, the guiding role of librarians, teachers and parents is essential to the successful implementation of the programme. Some themes explored in a given book will be unsuitable for particular readers. Some literary devices used in a given book will not be appreciated fully by students who can otherwise follow its plot. Some books will simply be too long for particular readers, even if they are able to access shorter texts of a similar level of complexity. No formula could possibly identify all the variables involved in matching the right books with the right child.

Understanding alternative reading levels

We understand how frustrating it can be to find that the ATOS level of a given book might vary from those determined by other objective or subjective measures (such as that indicated by the publisher). Alternative reading levels are calculated in different ways and draw on different data to determine their results. Here are a few factors to consider in terms of understanding such discrepancies:

  • Does the alternative book level reflect the consistent use of a transparent and well-established formula for determining text complexity?
  • Does the alternative book level take into account the text of the entire book, like ATOS does, or just a sample?
  • Does the alternative book level take into account the difficulty level of the words used, based on the largest available data set of words in books actually read by students?

We acknowledge that other formulae can produce different results, but because every book with an Accelerated Reader quiz has been levelled by the same ATOS readability formula, it remains the most consistent, and we contend the most accurate, way to compare books in the programme.

Calculating reading levels with ATOS

Although developed by Renaissance Learning for use with Accelerated Reader, ATOS is an open standard. The ATOS level of any text can be calculated using a simple online tool, the ATOS Analyser.

Further reading


Cecelia Powell
Managing Editor

Part of the original UK quiz writing team, Cecelia has nearly 15 years' experience of producing content for Accelerated Reader. Now the Managing Editor, she oversees both the editorial and administrative aspects of the Content department.


Posted on 23 June 2016 at 9:40 am
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