The Wisdom of Crowds

Condorcet’s jury theorem

An interactive lesson with 6 steps, about 5 minutes: you predict, drag and solve, with instant feedback on every answer. It runs in your browser and needs JavaScript.

What you’ll learn

• If each voter is right with p > ½ independently, the majority does better.
• Reliability approaches certainty as the group grows (41 voters at p = 0.6 give 90%).
• Correlated opinions or p < ½ destroy the effect.

Steps

1. Many imperfect judges
2. Three jurors
3. How big a jury?
4. Worse than a coin
5. The hidden assumption
6. Five good judges