Someone chose to pick Google's brain by testing and comparing search results based on keyword density or frequency, to see which ranks better.
As you can see, pages rank in order by the number of times a keyword is repeated (9x ~ 12x). Besides page size (number of words on a page), and keyword frequency, everything about those pages are identical, including PageRank.
Not so fast say others, including Bill Slawski, who pulls out patents data. Others point to recent information by Matt Cutts on keywords that target the long tail and Mike Grehan on latent semantic indexing.
In other words, when you think you have it figured out, you discover you're not finished yet.
Cre8asiteforums discussion restarts here: Keyword Density