tedster - 5:39 pm on Dec 27, 2011 (gmt 0) [edited by: tedster at 4:42 pm (utc) on Jan 11, 2012]
We've already had some discussions about this - here's one from September: [webmasterworld.com...]
There's not a lot of solid information from Google about the exact "how". My assumption is that it works something like the widely used Flesch-Kincaid reading ease assessments (used in MS Word). That would mean important factors are average number of words per sentence, and average number of syllables per word.
[edited by: tedster at 4:42 pm (utc) on Jan 11, 2012]