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The Flesch-Kincaid index is a measure of the grade level of a particular
piece of writing. I guess that you could consider it also a type of
readability index. The formula is 0.39 X the average number of words/sentence
plus 11.8 X the number of syllables per word.
The Gunning Fog index serves a similar purpose, displaying the requisite
grade level one must have reached in order to understand the information.