Text complexity and ATOS


The ATOS readability formula is a research-proven tool to guide 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?

Read the study »


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. »