Is there any consensus as to how many words are in the average adult's vocabulary? Over the years, I've come across various factoids and blurbs online and in magazine articles that have made statements like, "The average adult has a vocabulary of 20,000 words."
And if there is a consensus, how was such a number arrived at? Is the "20,000 words" claim (or whatever it happens to be) just a guesstimate or are these numbers arrived at in a more mechanical and, ideally, reproducible procedure? Have then been any formal studies in this vein?