The Keyword Effectiveness Index (KEI) calculates which keywords can be used to make a website SEO-proof. The KEI is a useful tool for writing good SEO texts, but you cannot extract any hard data from it.
For a good KEI calculation, it is important to look at the meaning of keywords and whether these keywords are also used by competing websites. The right keyword is a word that is often searched for in the search engine, but for which there is little response within the existing SERPs.
The KEI can be easily calculated using this formula:
KEI = the number of searches / the number of SERPs
The number of searches for a certain keyword can be found out via for example Semrush (indicated with 'search volume'). The number of SERPs can be found out by typing the keyword into the search engine. At the top it will say 'approximately ... results'.
The disadvantage of the KEI is that the quality of the competitors is not included. To prevent this, you want only the keywords mentioned in the title tag to be displayed in the results. You can easily do this by typing 'allintitle: <searchword>' in the search engine. If the keyword is mentioned in the title tag, you can assume that this is a competitor.
Furthermore, it is important to use not only short-tail search terms in the KEI calculation, but also long-tail search terms. This will give you a more specific insight into which keywords give the best results for search engine optimisation.