1999 on: nflverse game data (interim coaches appear under their own names). 1920–1998: head coaches from each team-season's Wikipedia article (CC BY-SA 4.0), matched to FiveThirtyEight's game results; a mid-season change is split by the records listed there. AFL games count; AAFC games don't. Team Elo (1999 on): 1500 is league average.