KEI stands for the Keyword Effectiveness Index. The formula provides an idea on how effective a specific keyword would be if you were to optimize pages for it. The formula for KEI is
where GS stands for the number of global monthly searches and BC – for the bid competition on Google Adwords.
The higher the KEI ratio, the more popular your keyword is (more people are searching with that keyword), and the less competition it has (fewer website owners are bidding for it in Adwords). Consequently, the keywords with higher KEI should be better to use for optimization, unless those keywords are not likely to convert visitors into paying customers or whatever other goal you want from site visitors.
The KEI is evaluated as follows:
fair: 0 – 4.99
moderate: 5 – 9.99
good: 10 -19.99
excellent: 20+