Resolved · 16 May 2026
Where should I host my Eurovision party?
A Denmark-themed party: commit to a premium venue (worth +€500 if Denmark finishes top-5, −€500 if not), or host at home. Before the final — forecast pulled from the production API on 2026-05-12, four days out:
Claude + Internet → book the venue.
Polymarket’s 66.5% is above the 50% breakeven, so expected value looks positive (+€165).
Claude + Credence → host at home.
Credence’s calibrated mean was actually higher — 72.7% — so on the point estimate it agreed: book. But with only n_effective ≈ 5 the posterior was wide; its 10th percentile sat at 45.9%, below breakeven. The robust call was to pass.
Why it mattered: Credence wasn’t “more right” about the odds — in fact it leaned more toward Denmark than the market did. The flip came from n_effective flagging that the evidence strength was too thin to bet on. The downside the wide posterior warned about — which even Credence’s own mean put near 1-in-4 — is the one that landed. The price said book; the confidence said don’t; the result said don’t.
Driving market: Denmark top-5 at Eurovision 2026. p_market 66.5% · p_calibrated 72.7% · n_effective 4.80, pulled from the production API 2026-05-12 — before the 16 May final. Outcome real (Denmark 7th); the venue payoff is an illustrative scenario. Not advice.
Show Claude’s math
EV (venue − home | p) = 1000 p − 500 ; breakeven p = 50%.
Market mean 0.665 → +€165 (venue). Credence mean 0.727 → +€227 (still venue).
Posterior Beta(3.49, 1.31), n_effective 4.80 → P10 = 45.9% → EV at P10 = −€41.
Robustness rule (commit only if P10 ≥ breakeven) → home.
The mean alone never flips this; the evidence does.
Assumption: 10th-percentile robustness rule.
Realized 2026-05-16: Denmark 7th, not top-5 → venue −€500, home €0.
The mean alone never flips this — and here the mean was on the wrong side. The evidence is what saved it.