How much better do groups of amateurs beat professional forecasters?

As it turns out about 30% - but as it turns, out the reason is because they have "doubt" (WSJ):

The top 2%, whom Prof. Tetlock dubs “superforecasters,” have above-average — but rarely genius-level — intelligence. Many are mathematicians, scientists or software engineers; but among the others are a pharmacist, a Pilates instructor, a caseworker for the Pennsylvania state welfare department and a Canadian underwater-hockey coach.

The forecasters competed online against four other teams and against government intelligence experts to answer nearly 500 questions over the course of four years: Will the president of Tunisia go into exile in the next month? Will the gold price exceed $1,850 on Sept. 30, 2011? Will OPEC agree to cut its oil output at or before its November 2014 meeting?

It turned out that, after rigorous statistical controls, the elite amateurs were on average about 30% more accurate than the experts with access to classified information. What’s more, the full pool of amateurs also outperformed the experts.

The most careful, curious, open-minded, persistent and self-critical — as measured by a battery of psychological tests — did the best.

“What you think is much less important than how you think,” says Prof. Tetlock; superforecasters regard their views “as hypotheses to be tested, not treasures to be guarded.”