The Dale–Chall readability formula is a readability test that provides a numeric gauge of the comprehension difficulty that readers come upon when reading a text.
It uses a list of 3000 words that groups of fourth-grade American students could reliably understand, considering any word not on that list to be difficult.
The formula was inspired by Rudolf Flesch's Flesch–Kincaid readability test which used word-length to determine how difficult a word was for readers to understand.
The Dale-Chall Readability Formula was originally published in their 1948 article A Formula for Predicting Readability[2] and updated in 1995 in Readability Revisited: The New Dale-Chall Readability Formula, which expanded the word list to 3,000 familiar words (text file version).
Regular plurals of nouns, regular past tense forms, progressive forms of verbs etc.