What is a Brier score?
A Brier score is a forecaster’s report card in one number: the gap between how sure you said you were and what actually happened, squared, then averaged over all your calls. Zero is perfect. Lower is better.
(Here for curling results? Different Brier. Good luck out there.)
The mechanics fit in a sentence. Say you’re 70% sure something happens. If it does, your gap is 1 − 0.7 = 0.3, and the score for that call is 0.3² = 0.09. If it doesn’t, the gap is 0.7, and the score is 0.49. The squaring is the part with teeth: being confidently wrong costs you far more than being humbly wrong ever saves you.
One number on the scale is worth memorizing: 0.25. That’s what you score by shrugging 50% at everything — the mathematical price of refusing to have an opinion. Beat it consistently, over enough questions, and your sureness is demonstrably worth something. That’s not a vibe; it’s arithmetic.
Inside that one number live two different talents: reading questions well, and knowing how sure to be — matching your 70%s to a world where they come in about 70% of the time. That second skill, the trainable one, is calibration, and the raw material it needs is exactly what a record keeps: every call, at what confidence, and how it landed.
Which is the point of keeping score at all. A single good call proves nothing — a coin gets those. A long record scored this way is the one thing luck can’t fake.
Quick answers
What is a good Brier score?
Lower is better: 0 is perfect. Calling 50% on everything scores exactly 0.25, so staying meaningfully under 0.25 across many questions means your confidence is carrying real information.
How is a Brier score calculated?
Turn your confidence into a number between 0 and 1, subtract what happened (1 if it did, 0 if it didn't), square the gap, and average across your calls. Say 70% sure and it happens: (1 − 0.7)² = 0.09. Say 70% and it doesn't: 0.49.
Is a Brier score the same as calibration?
Related, not identical. Calibration asks whether your 70%s come in about 70% of the time; a Brier score folds that together with how decisive your calls were into one number.