The Truth About Keyword Density
If you are chasing some magical keyword density in the hope that it will improve your search engine rankings you're only chasing your tail. Although the page should be self descriptive and the target keyphrases present in all the appropriate places, keyword density is simply a myth.
The protagonists responsible for keeping the keyword density myth alive tell us: "Search engines count how many times a term appears in a document. The more times a term appears, the higher the keyword density. The higher the keyword density, the more relevant a document becomes for that term."
What makes this claim believable is the logical flow of the statement, it sounds right. Then there is the mathematics used to convert those who remain agnostic or unbelieving.
The Keyword Density calculation
The keyword density of term i (KDi) = the frequency term i occurs in a document (TFi) divided by the total word count of the document (WCi). Thus, the keyword density of the word 'optimization' when repeated 4 times in a 100 word document would be: 4/100 = 0.04: giving a keyword density (KDi) of 4%.
This calculation only serves to make the notion of keyword density more believable. So much so that forums, article sites and blogs are full of posts proclaiming some mythical keyword density that will have customers flocking to your site and make you rich beyond your wildest dreams... Total Crap!
Keyword density, as described above, is a simplistic on-page measure that expresses a crude term/document ratio. This ratio fails to take into account the keyword density of any other document that may be competing for the same term. In an attempt to overcome this obvious inadequacy, you then compare the keyword density of your document to that of the top ranking documents and adapt your page to match the keyword density of that document set. The result: Spamglish!
How to screw up your pages, destroy your credibility, aggravate your visitors, and justify the necessity for the back button.
I think we have all seen our share of pages that have undergone this process. Pages become difficult to read, look spammy and lower the reader's perception of the authors writing abilities. Ultimately, credence and credibility suffer along with trust and conversions.
If search engines derived document relevancy based heavily on keyword density, then all you would have to do is repeat your target term over and over to get pages to rank. Search engines are not that dumb. Keyword density ratios fail to take into account the relative position (contextual relevancy) and relative dispersion (distribution) of terms in the document or how many documents are relevant for the term.
Furthermore, keyword density ignores internal linking, site structure, back links, how long users stay on the page, domain age, etc. Keyword density is a worthless metric; keyword density tools are a waste of time, and people who chase some mystical on page keyword density are probably doing more harm than good.
Write for the reader NOT search engines
If you write naturally with the goals of the reader in mind, contextual relevancy and keyword distribution will pretty much take care of themselves. Write for humans not Search Engines.