Measuring and comparing the demand for your keyword (Daily World Searches) against the number of Web pages that include it (Competition) is a helpful approach to pick up the best keywords. The resulting ratio, KEI or ‘Keyword Effectiveness Index’ will give an idea of how important a specific keyword is.
The formula for KEI is KEI = (DS^2/C) = (DS/C * DS)
DS = Number of daily world searches
C = The competition.
The KEI range goes from 0 to over 400.
< 0,001 = Poor keyword
0,001-0,010 = Good Keyword
0,010-0,100+ = Excellent Keyword
Keywords with the highest KEI combine popularity and less competition. It means that it is more likely that you will get a high ranking, if you use these keywords for optimizations.