The best football numbers are hard to get. Teams build their own models and keep them in the building. Many of the best public ones sit behind a paywall, and most of what is free is a box score or a stat that does not ask who a team played. I started Held Out because I wanted anyone to be able to see frontier advanced stats: ratings that adjust for the schedule, that pull a passer apart from the team around him, and that say how far to trust them.
The name is the one rule the site keeps. In statistics, held-out data is data a model never saw while it was built. Every number here is scored that way: fitted on earlier seasons, then tested on games it had not seen. An idea that did not help on those games did not ship, however good it sounded.
It is all built from free public data, most of it from nflverse, and it stays free: no paywall and no account. Kickoff weather, which sets how many points a game's projected score expects, is from Open-Meteo (CC BY 4.0): its historical archive for games played and its forecast for games to come. How RICE and BRADY work, and how they stack up against other metrics, is in the Methodology.