This is a simple but efficient method to approximately calculate the users' vocabulary level.

It is based on the subtitles of the Friends series, although it can be easily extended to other corpuses. After some simple pre-processing steps, the unique words are sorted according to their frequencies. Then, a binary search approach is adopted to find the vocabulary level of the users. This approach is based on the following simple rules:

• If the user knows the meaning of a word, it is likely that she/he also knows the meaning of more frequent words.
• If the user does not know the meaning of a word, it is likely that she/he also does not know the meaning of less frequent words.

Although these rules are not completely true, they are approximately acceptable. Anyway, these simple rules help us to obtain a very efficient method which only needs log2(n), so for example with a list of 10000 words you would need to answer about 13 questions.