How is the difficulty of a text calculated?

The difficulty is calculated by a combination of:

- Automated Readability Index: wikipedia article
- The percentage of words which are in the top 2000 most frequent words in the language. The majority of the word frequency lists are based on movie subtitles and come from this site: Invoke IT Word Frequency Lists

It's far from perfect, but seems to give somewhat plausible results for Spanish, English, French and German texts.

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