Taking KEI with a Grain of Salt
KEI, or Keyword Effectiveness Index, is a value often used in SEO circles to compare the value of potential key terms. It is the popularity of the term divided by the amount of competition. It makes logical sense to use this to decide on the best key terms because you ideally want high-traffic, low-competition terms that will generate a great number of qualified visitors without a lot of effort.
However, it is important to consider factors other than just popularity and competition when it comes to selecting key terms. For example, the quality of the competition can be just as important as the quantity, and sometimes even moreso. The number of pages that compete for the term in question is only a rough estimate as to the resistance you might expect to achieving high ranking. In other words, the correlation is not necessarily one-for-one between search terms.
Quality of the traffic is equally as important. Give some thought as to who might search for the key term you’re considering and why. Is this the sort of person who you would expect to convert on your website? Does the key term properly target your demographic, or is it likely to be searched for purposes completely unrelated to your site topic?
The point of the exercise is not to trust your metrics blindly. A key term may have a great KEI value but have no practical value to your website. Do the research, think things out, and use measures like KEI as a guide for decision-making, not as the sole basis.