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Keyword density today is somewhat of a myth. If it is natural, then Google is very forgiving of keyword stuffing, and they have pretty good algorithms to determine what is natural or not. With that in mind, the SEOmoz On page tools should be considered more of a guide than an absolute rule. I could go on, but these two videos say it better than I could: http://www.seomoz.org/blog/advanced-onpage-optimization-whiteboard-friday http://www.seomoz.org/blog/the-new-onpage-optimization-whiteboard-friday