To estimate the difficulty of a text, readability formulas count what is easy to count at the level of individual words and sentences. Typically, they use the length of a word or sentence as the indicator of difficulty. Readability formulas assume longer sentences are harder sentences. The formulas assume longer words are harder words.
The formula counts: number of letters or syllables in a word (readability formulas consider difficult words as words with three or more syllables).
Each readability formula measures text differently, but each formula typically assesses text only at the level of individual words and sentences, in a purely mechanical, objective way. They measure certain attributes of words and sentences in isolation, ignoring other important attributes at the sentence level, including how the writer has connected sentences into paragraphs.
1) Readability formulas consider longer sentences as more difficult to read than shorter sentences; however, sentence length is not the only attribute that can make a sentence hard to read. Syntax and cohesion can make a sentence difficult to read as much or more than sentence length.
2) Short sentences may be easy sentences in isolation, but they sound choppy and lack cohesion when the writer puts them together in a paragraph.
3) The downside of readability formulas is that they focus narrowly on individual words and sentences. The formulas ignore everything else that contributes to ease of reading and comprehension, including the active role of the reader. A readability formula that counts syllables and length of sentences does not consider the knowledge, life experience, literacy skills, and active search for meaning that individuals bring to the task of reading.