---
title: "What Is a Brier Skill Score?"
term: "Brier skill score"
type: glossary_term
canonical: https://www.moonwire.org/insights/glossary/brier-skill-score.html
---

# What Is a Brier Skill Score?

> How much better a set of probability forecasts scored than a simple reference forecast. Positive means better than the reference, zero means no better, negative means worse.

The **Brier skill score (BSS)** converts a [Brier score](https://www.moonwire.org/insights/glossary/brier-score.md) into a statement about added value: did these forecasts beat a naive alternative that required no skill?

## The formula

**BSS = 1 − (Brier score of the forecasts ÷ Brier score of the reference)**

- If the forecasts and the reference score the same, BSS = **0**: no skill shown.
- If the forecasts score better (lower), BSS is **positive**. A BSS of +0.10 means the forecasts cut the reference's squared error by 10%.
- If the forecasts score worse, BSS is **negative**. There is no floor: a confident forecaster who is often wrong can go well below zero.
- A perfect set of forecasts would score **+1**.

## Choosing the reference

The reference is what makes the number honest. The most common choice is the **climatological** or [base-rate](https://www.moonwire.org/insights/glossary/base-rate.md) forecast — always predicting the long-run frequency of the event. If Bitcoin has closed higher over seven days 56% of the time, the reference says "56% up" every week. Beating that is harder than beating a coin flip, and it is the right bar: a forecaster who cannot beat the historical frequency is adding noise, not information.

## Sample size matters

A skill score from a handful of forecasts can swing from strongly positive to negative with one or two outcomes. That is why a skill claim needs a sample size stated beside it, and why a result on a short sample is described as *provisional*. The same score on 200 resolved calls means far more than on 15.

## In MoonWire analysis

Our [daily prognosis](/insights/prognosis-daily-2026-09-09.html) reports skill against the base rate with the number of graded calls behind it — for example "a provisional +0.0426 on 18 graded calls" — and states when the sample is still below the threshold at which we would treat the figure as established. When the skill score is negative, the article says so. [Calibration](https://www.moonwire.org/insights/glossary/forecast-calibration.md) is reported separately, because a forecaster can have positive skill and still be over- or under-confident.

## Further reading

- [Wikipedia: Brier score (skill score section)](https://en.wikipedia.org/wiki/Brier_score)

## Where this appears in MoonWire analysis

- [Prognosis daily - rotation holds, three slots stay open, and the one call we refused](https://www.moonwire.org/insights/prognosis-daily-2026-09-09.md) — Sep 11, 2026
- [Prognosis daily - 29 Aug: one fresh Solana lean, the rest of the book left running](https://www.moonwire.org/insights/prognosis-daily-2026-08-29.md) — Sep 11, 2026
- [Choppy holds, breadth splits down the middle, and two calls resolve against us](https://www.moonwire.org/insights/prognosis-daily-2026-08-16.md) — Sep 11, 2026
- [Week-ahead read: a thin news window, a choppy tape, and four calls that all landed](https://www.moonwire.org/insights/prognosis-daily-2026-08-09.md) — Sep 11, 2026
- [Macro takes the wheel: a weak jobs print, an unchanged policy rate, and a tape that keeps thinning](https://www.moonwire.org/insights/prognosis-daily-2026-08-08.md) — Sep 11, 2026

---

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