Measuring and comparing the demand for your keyword or key phrase against the number of web pages that include it (your competition) is a recommended approach to detect the most efficient keywords to use throughout a website.
The resulting ration value is deemed the Keyword Efficiency Index, or KEI, and it provides a rough idea of how effective a specific keyword or key phrase will be.
The forumula for KEI is as follows:
KEI = (MS/30)2 /C\r\n
Typical KEI ranges from 0 to over 400.
The higher the KEI ratio, the more popular your keywords are (more people are searching using those keywords), and the less competition they have (fewer websites that would come up as relevant results in search engine results pages).
Keywords with the highest KEI combine the most popularity with the least competition.