FleschKincaid readability tests
Gunning fog index
Automated readability index
DaleChall readability formula
Spache readability formula
Fry readability formula
They all use words per sentence, along with syllables or letters or long/unfamiliar/complex/difficult words per word.
That seems to me overly simplistic, because it does not involve some estimate of syntactical complexity. To see why that is a problem, let us consider these three sets of sentences.
- I watched the cat. She was eating her dinner.
- I watched the cat, and she was eating her dinner.
- I watched the cat, as she was eating her dinner.
A complexity-based index would place (2) as not much more complex than (1) and (3) as significantly more complex than (1) or (2).
I went over to scholar.google.com to look for attempts to use syntactic complexity in readability testing, but I found only a few papers, and they did not state their results very clearly.