Development of the ATOS Readability Formula

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Accelerated Reader helps teachers accurately match books to students. The accuracy of this match is due in part to the process that is used to analyze the readability of books, the ATOS Readability Formula. This document explains the science underlying this process as well as three recent key enhancements to the formula: Expand the Graded Vocabulary List-from nearly 24,000 words to more than 100,000; optimized grade-level equation that increases validity at the lowest and highest ends of the spectrum, also allowing the ATOS Readability Formula to take full advantage of the new Graded Vocabulary List; and refined formula to estimate readability levels for short passages (of 3 or fewer sentences) with increased precision.

AUTHOR: Michael Milone.

The full report is available online.

Publication Date:

Updated: 06/30/2014

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