SMOG (Statistical Measure of Gobbledygook) is used to measure the readability of documents that have 30 or more sentences. The score is based on the number of "big" words (words with three or more syllables) in three-10 sentence passages. SMOG measures a document at a minimum of a fourth grade reading level.
Fog is used to measure the readability of documents that have less than 30 sentences. The score is determined by the average sentence length and percentage of three-or-more-syllabic-words in the entire passage.
Fry measures readability of a document based on the average number of sentences and syllables of three-100 word passages.