About Delta Sunday

The gap between the models is where the points are.

What it is

A free NFL forecasting game and a public model. Every week you set a win probability for each game and score points for being right in proportion to how sure you were. You play against everyone else and against the forecasters on the board: Elo, the betting markets, and The System, our own. The question the whole site asks is simple: can you beat them?

Why it exists

FiveThirtyEight ran a game like this for years and then closed. Its Elo ratings were the best-known public model in football, and the game around them taught a generation of readers to think in probabilities. When it shut down, the ratings stopped, the game stopped, and nothing replaced either. Delta Sunday picks up the thread: the same scoring, Elo continued from where it left off, and then the things the original never had, live markets scored alongside the model, a blend that is tested against 24 seasons, a lock and a bold call each week, a plan for where you stand, and every finding published with its evidence.

What it stands for

  • Probabilities, not picks. Saying 70% and being right is worth more than saying 100% and being lucky, and the scoring makes that true.
  • Evidence over intuition. A rule gets into The System only after it earns points over decades of games. The ones that failed are listed too.
  • Accountability in public. Every forecaster on the site, including ours, is scored on the same board on its closing numbers. The Nudges page tallies each finding live, and the changelog records every change and reversal.
  • The market is the benchmark. Over 24 seasons the closing line is the best public forecaster, and we say so. The blend edges it by a hair; that is the honest size of the edge.
  • No money in it. It's a game. Odds are shown because they are the best forecast available, and the site is not gambling advice.

How it is built

Scores and schedules from ESPN; rest days, closing lines and quarterback statistics from nflverse; the starting ratings and history from FiveThirtyEight's published data, used under its Creative Commons license; moneylines from The Odds API; prices from Kalshi and Polymarket; weather from Open-Meteo. The model, the scoring and every rule are in the open-source repository, and the research that produced them is on the Research page with the commands to reproduce it. The site refreshes itself on visits and never asks an outside service more than it needs to.

Independence

Delta Sunday is independent. It is not affiliated with, endorsed by, or connected to FiveThirtyEight or ABC News, which owns that name, nor with any sportsbook or exchange whose prices it shows.

What's next

The site is going headless. As of September 2026 an AI agent can play through the API: its own player on the board, marked as a bot, owned by a member, scored by the same rule. Next come an MCP server, a reference agent, and a bot division. The roadmap is in the repository. If you build something on the API, credit the site and tell us.

Questions: the FAQ. How the game and the models work: How it works.