Reading their paper, I learned something about my own text. In footnote 17, they tell us the following:
Standard tools originally developed to compare the complexity of the language in training manuals in the US Navy are used to compare the readability of the textbooks. The result of the Flesch test is that the CORE text is somewhat more complex than Mankiw’s but less so than Krugman-Wells and Samuelson 1948. The tests are based on syllables per word / proportion of multisyllable words, and sentence length. The use of multi-syllable words is virtually the same across the four texts, but Krugman-Wells and Samuelson use longer sentences. The F-K measure’s output is the US grade level needed to comprehend the text, according to which, Samuelson 48 and Krugman-Wells are comprehensible to a 12th grade student, Mankiw to a 10th grader, and CORE to an 11th grader.I was pleased to learn this, as I try to write in shorter sentences to make the text more readable. Learning economics is hard enough. So the style of writing should be as accessible as possible.
Addendum: For comparison, according to this source, academic papers are written at about the 12th grade level. Malcolm Gladwell writes at the 9th grade level, F. Scott Fitzgerald at the 8th grade level, Stephen King at the 6th grade level, and Ernest Hemingway at the 4th grade level. It also says that only about 1 in 8 U.S. adults can read at the 12th grade level.