The Eagles are currently favored on Sunday by a point and a half.
How accurate is that?
Using data from sportsoddshistory.com (I believe these are all closing spreads we’re using), this is what we found:
Over the last fifteen seasons, including this one, the average regular season spread has missed the final margin by a magnitude of 10.3 points. Over the last fifteen seasons, including this one, the average playoff spread has missed the final margin by a magnitude of 9.6 points. Over the last fourteen Super Bowls, the average spread has missed the final margin by a magnitude of 11.4 points.
Switching to medians, rather than using averages that are stretched by games like the one in 2020 where the Patriots beat the Chargers 45–0, the error margin of spreads is 8.5 in the regular season and 7.75 in the playoffs, meaning a little more than 50% of regular season spreads have been within 8.5 points of the eventual margin and exactly 50% of playoff spreads have been within 7.5 points of the eventual margin. (The Super Bowl median is also 7.75.)
So, the answer to the question of how accurate Super Bowl spreads are and how accurate NFL spreads are in general is that the baseline miss is about eight points. That’s normal. But for a better look at the shape of those misses…
That’s a little more than two thirds of regular season spreads missing by 12.5 points or fewer, and roughly one ninth of regular season spreads missing by more than 21 points in either direction. In the playoffs, the portion within 12.5 points of the exact margin rises to roughly four in five, while only one in thirteen spreads over the last fifteen postseasons has missed by more than 21 points. Oddly, medium misses are more common when underdogs cover the spread than when favorites do, for reasons that are unclear but may have to do with coaches making touchdown/field goal decisions in attempts to win and therefore pushing the target score slightly to the side of the underdog.
To bring this all back to Kansas City and Philadelphia, what we’d generally expect is a 50% chance that the game is somewhere between Philadelphia winning by nine and Kansas City winning by six, with roughly a 75% chance it’s somewhere between Philadelphia winning by fourteen and Kansas City winning by eleven and about a 90% chance it’s somewhere between Philadelphia winning by 22 and Kansas City winning by 19. That’s not exact—alternative spreads could give you a better idea of the breakdown—but if you’re looking at the odds on the bottom line and wondering what they can tell you about the game you’re about to watch on Sunday, this is the broad picture.
