The keyword density tool is a standard part of any search engine optimization tool arsenal but how important are the results of the tool for determining how well a page is optimized? What role does keyword density play in the search engine algorithms?
First, as far as we know, keyword density has never been the formula used by search engines to calculate keyword relevance for a page. Unfortunately, we don’t know what the true formula is (it’s a secret and Google won’t tell).
Most believe that it is actually based on term weight and that would probably be a better way to evaluate keyword use on a web page. However, one of the main components for calculating term weights is the total number of times the keyword is used on the internet. We have no way of knowing what that number is because even if we could crawl the whole internet and find all the pages and all the words, we still wouldn’t know if our crawl was the same as Google’s or Yahoo’s or any other search engine. This is why we can’t create a tool to more sharply evaluate keyword use on a page using term weight — we don’t have access to all the data. (And it may also explain part of the reason why you see different results across search engines — they have different indexes).
However, there are some things we do know:
The number of times that the key phrase (or parts of it) appears on the page carries some weight. We know this because we have seen rankings change after keywords are added to pages. We have also observed sites undergo “penalties” for stuffing keywords into meta tags, alt attribute tags or in fine print at the bottom of the page — commonly referred to as “keyword stuffing”. Too many unnaturally placed keywords may trigger a spam filter and cause rankings to go down. This appears to be algorithmic so we have to assume that counting the keywords on the page has some place in the algorithm, however small.
The total amount of page content carries some weight. Pages with more content tend to rank better (all other things considered).
Overall keyword theme relevance carries some weight — search engines evaluate the content based on factors like proximity of the keywords to each other, ordering on the page, position in the overall body content, positioning in important places on the page, as well as the thematic relationship between the key phrase and other words on the page. In some cases, synonyms of the key phrase can even count toward keyword relevance. So, the algorithm is far more complex than a simple keyword density calculation, but keywords undoubtedly do feature in the calculation.
Finally, we know that if search engines cannot properly interpret the code on the page, they cannot “read” the content properly. This could result in parts of the code being included in the text indexing for the page possibly resulting in lowered overall keyword relevance for the page. Search engines might even have trouble seeing the keywords on the page which could affect any keyword weighting for relevance. While it is true that they likely attempt to filter out this kind of noise in the indexes, we do not recommend leaving it to the robots — it’s best to control what they see whenever possible.
So, we use keyword density analysis tools as a quick way to look at the content on websites the way that a spider might see it. If the keyword density is too high, it can be a signal that the page may trigger a spam filter. If the keyword density analyzer doesn’t find the keyword at all or the density is very low, it can indicate that the page is not appropriately targeted. Because the keyword density tool is basically a simple spider, if it cannot properly “read” the content, it could indicate problems with the coding on the site.
That said, if the content on the page is properly focused on the keyword theme of the page and the keyword appears in all of the important places, title tag, description tag, headline, a couple of times in the main text content (ideally in the first and last sentences), the text is probably fine even if you don’t get up to the “optimal” keyword density. Furthermore, if your key phrase is very long (more than 3 words), using it 4% of the time on the page would be very unwieldy so you may not even want to use the keyword that many times. Content should always be written for people first, with the keyword density tool just used as a kind of reality check to make sure that it’s also going to be okay for spiders. A real “optimized” page will have content that is useful for visitors and will attract quality inbound links.