Common Core Reading Standard 10 not only prescribes the difficulty of texts students should become able to read, but also the difficulty diet of texts schools should ask their students to read across the school year. The use of quantitative text-assessment tools in the implementation of this standard warrants an examination into the validity of that use. To do so, we concentrate entirely on the criterion variable that ultimately is the goal of reading instruction and learning: reading comprehension performance. We examine whether the comprehension criterion variables for today's quantitative tools validate how their text-difficulty estimates are being used. We conclude that the Common Core State Standards' new text-difficulty grade bands are inadequate to serve as a criterion variable for quantitative text tools because the data on which these bands are based did not compare comprehension growth for various groups of students reading different difficulty diets over a school year.