KEI is an acronym for Keyword Effectiveness Index. The formula for KEI is KEI = (DS^2/C) = (DS/C * DS) , where DS is the number of daily world searches and C is the competition . The KEI ranges from 0 to over 400. KEI ( Keyword Effectiveness Index) will help you determine the best keyword suggestions possible and let you narrow down the list of keywords and key phrases to optimize your pages for. Google displayed result for the keyword divided by number of search for the keyword equals KEI. It measures the value of a search term. It factors in the amount of times a term has been searched for versus how many other web pages target the exact match phrase. The more is keyword effectiveness the more is its search value.
When using KEI, keep in mind that KEI is only an indicator and should not be the sole reason for choosing particular keywords.
I have googled this for you and find a helpful link for you: www(dot)seo-artworks(dot)com/university/GoogleKEI(dot)html
I think this site explains it pretty good: KEI SEO > http://keiseo.com/ I would judge your own interpretation more then KEI score though.
Some webmasters also use the Keyword Effectiveness Index (KEI) to determine the value of their keywords. The KEI compares the number of searches for a keyword with the number of search results to pinpoint which keywords are most effective for your campaign.