Quite simply it is the total number of vocabulary words in a book divided by how many thousands of words in that book. So the difficulty index for a 100,000 word book having a vocabulary count of 100 equals exactly one.

Remember that our difficulty index IS NOT our rating of how good a book is; we have read many fantastic books that do not require extensive vocabularies (Jane Harper's The Dry is one example, Fannie Flagg's Fried Green Tomatoes is another). If people thought that the best books were only those with the most difficult words, they would only read books in languages they don't understand.

It only makes sense to calculate this index for titles we list as comprehensive, so we do not publish difficulty indices for partial (or other) types.

Roughly means that we are only partway finished with our vocabulary collection; the value is calculated by extrapolation. For example, as of today our Hamlet vocabulary collection contains 202 words. Had we no more terms to add at this point the difficulty index would be:

However since this vocabulary collection is only approximately halfway finished: So when the collection is complete, Hamlet will likely have a difficulty index in the neighborhood of 13 (we should be finished with the Hamlet collection by spring of 2020).

About implies the collection is at least 90% complete. The difficulty index in these cases likely won't drop, but the value might inch up somewhat.

As of today it will likely be Shakespeare's Hamlet since Shakespeare liked to coin new terms. We decided to make Ben Jonson's The Alchemist comprehensive as well just for comparison.

As a general rule older titles tend to have higher difficulty indices.

It only makes sense to calculate a difficulty index for comprehensive collections, and since it is not practical for us to create such collections for every title it just cannot be otherwise.