Had call recently to cite the Gunning Fog Index as a reason not to go with some proposed copy.  What’s that? This:

It’s a formula by which to assess the ‘readability’ of any piece of writing. Essentially, the more long words, and the more words per sentence, the more difficult any piece of writing will probably be to understand. A sentence which scores a pretty fair 16.16, using the calculator kindly provided by Simon Bond, to save us wordy folk the headache of all that working out.