I think it's necessary to take into account the fact that some subjects might lie, or might really not know the number, and the words that are said might give precious (and even perhaps indispensable) clues about that. Remember, this is a psychology experiment, and, in a psychology experiment, you better take psychological factors into consideration. If you all agree that one (numerical) answer definitely seems more credible than the other (without knowing the correct number that the researcher viewed, of course), this could therefore be of great interest, I think. If you (members of this forum) don't know the correct number, you can't be biased.