“…When determining the difficulty level of written material, researchers and textbook publishers take an approach that is quantitative, qualitative, or a combination of both. The quantitative approaches traditionally reduce the level of text difficulty to a single metric on the basis of one or more measures of sentence complexity and vocabulary difficulty (e.g., Fry, 2002;Gunning, 2003;Peterson, 1991). More recent readability systems (e.g., Source Rater, Reading Maturity, Co-Metrix) include additional measures, such as text coherence (for a discussion, see Nelson, Perfetti, Liben, & Liben, 2012).…”