800 strangers guessing an ox's weight beat every individual expert's estimate

At a 1906 livestock fair in Plymouth, statistician Francis Galton collected 787 tickets on which fairgoers had guessed the weight of a slaughtered, dressed ox, expecting the average to be useless. The median guess came to 1,207 pounds against a true weight of 1,198 pounds, within about 1%, closer than any individual expert's estimate. The result helped found the modern idea that aggregating many independent guesses can outperform a single expert's judgement.

— Francis Galton, Wisdom of the crowd — Reported in Nature, 1907