If you ever find yourself reading an article on the importance of keyword density in SEO you can safely assume that the author doesn’t know what they are talking about. Keyword density is not a phrase or even a concept that is used by search engineers working for Google, Yahoo or MSN, it is simply a fiction invented by the lower echelons of the SEO community.
The importance of a keyword or a keyphrase on a page for any given query depends on the content in which the term is used not on keyword density or frequency counts. Search engines use term vector models which are mathematically complex and beyond the scope of this post. If you want to know more about term vectors then take a look at Term Vector Fast Track Tutorial.
Even knowing that search engines use term vector models is not going to help us in writing copy for our webpage. This is for two main reasons, firstly we don’t know exactly what model they are using and secondly in order to make any meaningful analysis it is necessary to perform calculations across every page on the web (which of course is what the search engines do).
So where does that leave us when it comes to writing copy for our webpages and incorporating our chosen keywords and keyphrases? The answer is simple, write your pages for the user and do not write what you think the search engines would like to see! Apart from writing good user copy the only consideration for keywords and keyphrases is to make sure that they appear naturally on the page.