Short answer: the KEI (Keyword Effectiveness Index) balances search volume against competition, helping you find keywords with high demand and low supply.
The reasoning behind it is useful: a keyword with high search volume but few quality pages is more attractive than a keyword everyone is already targeting. The KEI attempts to capture that ratio in a single number.
However, be cautious with the results. The formula looks at the number of competing pages, not how strong they are. Ten thousand weak pages are easier to beat than three pages from established players with a strong link profile. A favorable KEI can therefore point to a keyword that is unattainable in practice.
How we use it: as an initial filter to narrow down a long list, never as a final judgment. After that, we look at each keyword to see who is actually ranking, how strong their authority is, and whether the search intent matches what you sell. The latter carries more weight than any number.