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Edwords (ěd · words) n. 1. PreK-12 glossary breaking through buzzwords to solve the challenge of a common definition. 2. Renaissance® resource to help educators take part in discussion, debate, and meaningful discourse. 3. Educators’ jargon buster.

What is ATOS?

ATOS is a measure of readability—in other words, a readability formula designed to guide students to appropriate-level books. ATOS takes into account the most important predictors of text complexity—average sentence length, average word length, and word difficulty level. The results are provided in a grade-level scale that is easy to use and understand.

For example, a book with an ATOS readability estimate of 4.5 is written in a way that is understandable to individuals who have reading comprehension skills typical of a student in the fifth month of grade 4. A book with an ATOS of 6.0 would be understandable to individuals with reading comprehension skills of a typical student entering grade 6.

ATOS was originally developed by Renaissance in partnership with Touchstone Applied Science Associates, Inc. (TASA). ATOS is used in multiple Renaissance products, including Accelerated ReadermyON, and Star Assessments.

Is “ATOS” an acronym? What do the letters stand for?

Yes, “ATOS” is an acronym: 

  • The “A” stands for Advantage, because the formula was created in the late 1990s, when Renaissance was known as Advantage Learning Systems.
  • The “T” stands for TASA (now Questar Assessment), the group that Renaissance partnered with in the original development of ATOS, specifically for their experience in vocabulary and their graded vocabulary list.
  • The “O” and “S” stand for “open standard,” which means that rather than a propriety and monetized reading formula, ATOS is made available for free to all.

What elements are considered in assigning an ATOS level?

Different readability formulas include different elements in their calculations. Some common elements include sentence length, word length, word frequency, and vocabulary level. Also, some formulas analyze only a portion of an overall text, while others analyze the entire text.

ATOS was specifically created to overcome some of the shortcomings of older readability formulas. At the time of its development in 1998, the work around creating ATOS was believed to be the largest and most comprehensive study of readability ever conducted (Milone, 2014).

To determine the ATOS level of a text, Renaissance analyzes the entire text, and the following elements are considered:

  • Average word length
  • Average word grade level
  • Average sentence length
  • Book length

In comparison, the Lexile® formula specifically considers the following:

  • Word frequency
  • Average sentence length
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Are there different versions of ATOS?

Yes, versions of ATOS include ATOS for Books and ATOS for Text (for articles and shorter works). These are described in more detail below.

In 2010, we also introduced a Spanish version of ATOS.

Has ATOS been updated since 1998?

Yes. In early 2013, a variety of enhancements were made to the ATOS formula, most notably a four-fold increase from the original TASA graded vocabulary list of 24,000 words to an expanded list of over 100,000 words.

How does ATOS compare to other readability formulas?

ATOS performs as well as Lexile and other popular readability formulas. Driven by the CCSS and other standards’ emphasis on text complexity, a major evaluation of the most popular formulas was undertaken by Nelson et al. and included evaluation of the following: 

  • ATOS
  • Coh-Metrix Text Easability Assessor
  • Degrees of Reading Power: DRP Analyzer
  • Lexile
  • Source Rater (now known as TextEvaluator)
  • Reading Maturity Metric by Pearson
  • REAP 

Each formula was then used to rate a variety of passages, including the following:

  • Exemplar texts rated by education experts and published as Appendix B of the CCSS
  • State test passages
  • Passages from the Stanford Achievement Test (SAT-9)
  • Comprehension passages from the Gates-MacGinitie Reading Test
  • Passages from the MetaMetrics Oasis platform

The conclusion of this analysis was that “all of the metrics were reliably, and often highly, correlated with grade level and student performance-based measures of text difficulty across a variety of text sets, and across a variety of reference measures” (Nelson et al., 2012).

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How do I determine the ATOS level of a text?

To find the level of shorter passages (e.g., newspaper or magazine articles, reading passages, general classroom materials), use ATOS for Text.

To find the level of an entire book, first consult AR Bookfinder, which catalogues the titles (more than 220,000 and counting) supported by Accelerated Reader. If the book is not listed, use ATOS for Books. If the full book text is not easily available, you can enter a portion of the text and an estimate of the total number of words to receive a level using ATOS’s Estimated Word Count option.


Milone M. (2014). Development of the ATOS readability formula. Retrieved from:

Nelson J. , Perfetti C. , Liben D. , & Liben M. (2012). Measures of text difficulty: Testing their predictive value for grade levels and student performance. Retrieved from:

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