ATOS™

Readability formula for books

ATOS (Advantage-TASA Open Standard) is the readability formula developed by the makers of Accelerated Reader® reading software. Designed by a team of leading readability experts, ATOS includes statistics from actual student book-reading---over 30,000 students, reading almost 1,000,000 books.

• ATOS employs the three statistics researchers found most predictive of reading difficulty—words per sentence, characters per word, and average grade level of words. The ATOS formula determines average grade level based on a list developed from Advantage Learning’s word-frequency database.
• ATOS uses whole-book scans instead of text samples.
• ATOS takes book length into account, which readability formulas do not reflect.
• ATOS specifically addresses the challenges of leveling emergent-reader books. Researchers utilized the Reading Recovery® scale to identify key variables for leveling such books.