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Text complexity, ATOS, and Lexile® Measures

ATOS and Lexile measures are both valid and reliable measures of text complexity that provide a basis for matching students to reading materials. As with all readability formulas, the resulting value is an estimate of the understandability of the text. Accelerated Reader, myON Reader, Star Reading, and AR BookFinder display book level data in either ATOS or Lexile measures, based on the preference of your school or district.

Two ways to measure text complexity

What is a Lexile measure?

The Lexile Framework for Reading assesses both the reader’s level and the complexity level of the material being read. When these two measures match, a targeted reading experience occurs, resulting in growth. Renaissance has partnered with the creators of the Lexile Framework, MetaMetrics, Inc., to be able to bring Lexile Measures into Accelerated Reader and Star Reading. To learn more, visit their website to read about Lexile measures.

What is ATOS?

The ATOS readability formula guides students to appropriate-level books. Confirmed as a valid and reliable text complexity measure for the Common Core Standards, 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.

Why is text complexity important?

How is text complexity determined?

Adapted from Appendix A of the CCSS for
English Language Arts and Literacy.


Qualitative factors include levels of meaning, structure, language conventionality, clarity, and knowledge demands. Interest level (i.e., content and maturity level of text) and genre are provided in AR BookFinder.


Quantitative dimensions include word length or frequency and sentence length that can only be measured efficiently by computer software.

Reader and Task

Reader and Task considerations include students’ motivation, knowledge, and background interests along with the purpose and complexity of task assigned, and are best made by the teacher.

For more on text complexity's reader and task considerations, read this blog post.

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