This one was really interesting! (From the koohii forums, again. I'm not sure if I need to spend more time there, or less. So full of interesting ways to waste time.) It purports to be a test that can determine
roughly how many words you know in Japanese from a fifty-word quiz. (Brought to us by the publisher of an electronic CD dictionary, apparently.)
I was hoping the test was endlessly self-generating, but actually it seems like there are only three versions (too bad, it would be a fun way to test change over time). As far as I can tell from trying the first two, the tests are progressively harder, but interestingly, although I was able to identify far fewer words on the second one, both reported that I had a similar level of vocabulary: between 15,000 and 16,000 words. (Which is elementary school level, if you want a depressing statistic. Of course native speakers recognize all sorts of random words a foreign speaker typically wouldn't. University graduates allegedly can recognize something in the range of 40-50K words. I wonder how adult native speakers do on the test.)
I have no idea how statistically accurate the test is, but 15K sounds about right to me (maybe a tad high) - I have an anki deck of about 9000 vocab words that I routinely run through, and there must be a lot of words that aren't in my deck that I recognize just from casual reading.
I'd be really curious to know what my more fluent friends would score on the test!