Begeny, J. C. and Greene, D. J. (2014), Can Readability Formulas Be Used to Successfully Gauge Difficulty of Reading Materials? Psychology in the Schools, 51: 198–215.
Teachers, parents and textbook companies use technical "readability" formulas to determine how difficult reading materials are and to set reading levels by age group. This study from North Carolina State University shows that the readability formulas are usually inaccurate and offer little insight into which age groups will be able to read and understand a text. In the study, 360 students (grades 2-5) read six written passages out loud. The researchers assessed the students’ performance, giving each student an "oral reading fluency" score, which is considered a good metric for measuring reading ability. The researchers then used eight different readability formulas to see which level each formula gave to the six written passages. Results varied widely, with one passage being rated from first grade to fifth grade level. The levels assigned by the readability formulas were then compared with researchers’ assessments of each student’s actual ability to read the material. Seven of the eight readability formulas were less than 49 percent accurate, with the worst formula scoring only 17 percent accuracy. The highest-rated formula was accurate 79 percent of the time.