Keyword Effectiveness Index (KEI) attempts to characterize the value of a keyword based upon its popularity and the number of competing websites. Higher KEI values are "better" in that they represent more searches per website.

Typically KEI is the the ratio of the square of the searches upon a particular keyword in a day divided by the number of websites that are listed for that keyword. For example, a keyword that has 100 searches a day and for which Google shows 5000 websites would have a KEI of 2. (100 * 100 / 5000)