The NFL Edge Index tracks how teams perform against Polymarket odds, compounding into a rating across seasons.
HOW IT WORKS: Edge function pulls Polymarket's API for NFL game markets. Then we capture last valid price before kickoff (filtered 5-95% to exclude illiquid noise). After games finalize, it compares outcomes to pre-game odds. Each team starts at a baseline rating, and if they beat expectations as an underdog, rating climbs. Lose as a favorite, it drops. The rating compounds into a trajectory across seasons.
THE FINDING - NFL FAVORITES ARE OVERPRICED: 80%+ implied: 75 games, market expected 85.1%, actual 70.7% 70-79% implied: 161 games, market expected 74.7%, actual 67.1% 60-69% implied: 201 games, market expected 64.5%, actual 53.7%
Heavy favorites underperform by 14 percentage points. "Any given Sunday" is measurable.
For comparison, I ran the same analysis on 5,600 NBA games. Basketball markets are almost perfectly calibrated - 80%+ favorites win at ~82%. The NFL gap suggests something structural: parity, injury variance, weather, or thinner markets.
TEAM-LEVEL FINDINGS: Best underdog performers - Kansas City: 76.5% win rate as underdogs (13-4). Market priced them at 31% average.
Detroit Lions: 65% win rate as underdogs (13-7). Market consistently undervalued them.
Worst as favorites - Carolina Panthers: Lost 87.5% of games when favored. 14 losses in 16 favored games. New York Giants: 76.9% loss rate as favorites.
Biggest upset - Dallas beat NYG at 7.5% implied odds (Nov 2023). Market gave NYG 92.5%.
EDGE ACCURACY: 70.9% accuracy when model had 70%+ confidence (161/227 games).
Live dashboard: https://attena.xyz/nfl
If you're trading on Polymarket, I'd be curious if you find this data valuable.