What is a Brier score?

A grade for forecasters. 0 is perfect, 0.25 is coin-flipping. Lower is better.

Take your forecast as a decimal, subtract what happened (1 for yes, 0 for no), square it, average over all your calls. That's your Brier score, the standard grade in forecasting research.

For scale: flipping a coin scores 0.25, and Manifold's play-money crowd manages about 0.168. Skill shows up in the decimals.