Keyword density is the ratio of how many times that keyword or keyword phrases are used compared to all words including those keywords within that area.  A keyword density of 5% is considered the optimal target for keyword density on a page. 

Key factors that will effect keyword placement when considering keywords and what the search engine algorithms look for when ranking your page in terms of importance and relevance are prominence, weight, proximity, and frequency. 

Prominence is the closeness of the keyword to the beginning of any given area.  For Example considering your title tag and that when the first word of your title tag is your keyword it has a higher score from the algorithms than if you just put your company name or something else.  If you did not put it your keyword close to the beginning of you content then the keyword would be considered less important which could affect the rank you are given on a search engine. 

Weight or density is the ratio of how many times that the keyword or keyword phrases are used compared to all words within that area.  Keyworddensity.com is a free tool that you can use to find your keyword density and will give you a keyword density report.

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