At a county fair near Plymouth in 1906, 800 people participated in a competition to guess the weight of an ox. Statistician Francis Galton observed that the average of all 800 guesses was 1,197 pounds – it actually weighed 1,198 pounds. The guess of the crowd as a whole was far closer that that of any of the 800 individual guesses.
Risk assessment involves coming up with a value for the impact that a risk might have, were it to occur, and a likelihood or probability of it actually occurring. By getting the view of a group of people, the risk gets discussed and understood, and a realistic idea of impact and likelihood can be arrived at.
The Galton story was quoted frequently in the aftermath of the 2003 Columbia shuttle disaster where the risk assessment prior to launch had been restricted to a very few people.