The Victorian polymath Francis Galton noticed one day an interesting competition at his village fair:
the participants had to guess how much meat could you get from a beef that grazed in front of the crowd. 800 persons participated to the competition but no one guessed the right number (it was 1198 pounds), including several professional butchers.
Galton analyzed all the guesses and found out that their average was 1197 pounds, an error less than 0,1 %. Nobody was able to do better.
It was the year 1906. Galton’s finding remains true now: crowd wisdom beats the one from the experts.
Translating it into effort estimations: when they are coming from all the team members they are very often better than when they are coming from one “expert”.