Have you done any blue collar work? Depending on situation, many folks carry large thermos of coffee with them, and between morning and afternoon breaks, and lunch, quite a bit of coffee is consumed. Yes, this is my experience (and that isn't scientific), but simply stating that only office dwellers drink a lot of coffee isn't exactly scientific either.

That said, I agree with the sentiment here (and of the siblings) that controlling this sort of study is rather difficult.