Whenever I see these sorts of things I try to psych them out. In this case I deliberately thought of a number, discarded it, and repeated this process several times (rather than putting down 7, which was my first thought). So, unless you're trying to measure frequency of picked numbers when the pickers are trying to game the system, I don't think this will prove much. Nice interface though.
That's why one of the metrics is time, both from start of page to picking a number, then start to hitting choose. It's not perfect, but it'll hopefully be possible to differentiate between people who pick right away, and people who think about it.
