Research question and measurement
How does the accuracy of market-implied probabilities change as a prediction market approaches closure? The paper measures Brier loss at six checkpoints during the preceding 30 days, using Gamma closedTime as the operational time anchor.
The analysis keeps the same cohort at every checkpoint, applies a 12-hour price-freshness requirement, and averages markets within events before weighting events equally.
Findings and interpretation
Mean Brier loss falls from 0.088253 at 30 days before closure to 0.034962 at 12 hours before closure, a decline of about 60.4%. In the Murphy decomposition, increased resolution accounts for the largest part of the improvement.
These results describe forecast errors in a fixed sample. They do not directly measure objective uncertainty or information arrival, and they do not establish continuous or monotonic acceleration in the improvement of forecasts.
Paper details and original text
An English-language working paper published on SSRN. The date shown for this Research entry is the revision date.
| Item | Details |
|---|---|
| Original title | The Temporal Structure of Forecast Error on Polymarket: A Decomposition of Brier Loss over the 30 Days before Operational Market Closure |
| Author | Seiryu Ando |
| Date written | 2026-07-21 |
| First posted on SSRN | 2026-07-24 |
| Last revised on SSRN | 2026-08-15 |
| DOI | 10.2139/ssrn.7151878 |