Suggestion: Learned Words
Currently, learned words are limited to words focused on by Duolingo. Unfortunately, that doesn't give a very realistic impression of the number of words the user knows in the language. I think you should add another statistic that also counts utility words understood. By that I mean for example: "Je suis un acteur", given a focused word of acteur, should acknowledge that I must have also understood je, suis, and un as well. So I should have 4 understood words, and 1 learned words (to keep what Duolingo currently tracks).