Dale–Chall formula

The Dale–Chall formula is one of the ways to predict how difficult text is to read. It is a readability test. The Dale–Chall formula is a one-variable test: it measures the difficulty of the words used. It does not measure the difficulty of the sentence structure (the syntax).
One of the first readability tests, the Dale–Chall formula used a vocabulary list. It counted the number of listed words in a passage, and applied a formula which gave a grade level. It was used to rate textbooks for grade levels in US school districts. In 1995 the authors published an updated word list.
It is easy, in principle, to use a vocabulary list as part of a computer-based readability measure. The list is organised as a look-up table. The percentage of listed words in a passage gives the data for the formula, and the user is presented with a grade level.