Well Calibrated, Unevenly Priced: Market Calibration and the Favourite–Longshot Tilt in Online Fixed-Odds Betting

The favourite–longshot bias is among the most replicated anomalies in the economics of wagering markets, yet the modern evidence base still leans heavily on historical parimutuel horse-racing data. This paper tests the calibration of contemporary online fixed-odds markets using both sides of 18,238 settled two-outcome markets (36,476 outcome observations) across five sports between November 2024 and July 2026, with bookmaker margin removed from all quoted probabilities. Two findings emerge. First, de-vigged market probabilities are impressively well calibrated: in every probability decile, realized outcome frequencies fall within roughly two percentage points of vig-free implied probabilities. Second, the residual deviation is not noise but a systematic, antisymmetric tilt: outcomes priced below 50% overperform their vig-free probabilities by one to two points while favourites symmetrically underperform, and the tilt roughly doubles (±5 points) in player-proposition markets — the same market family that carries the highest measured margins. The pattern is consistent with bookmakers allocating margin disproportionately to longshot prices rather than spreading it evenly. Companion study and open data: https://betbetter.world/studies/market-calibration