How Our NFL Predictions Work – FiveThirtyEight

The Details

FiveThirtyEight has an admitted fondness for the Elo rating — a simple system that judges teams or players based on head-to-head results — and we’ve used it to rate competitors in basketball, baseball, tennis and various other sports over the years. The sport we cut our teeth on, though, was professional football. Way back in 2014, we developed our NFL Elo ratings to forecast the outcome of every game. The nuts and bolts of that system are described below.

Game predictions

In essence, Elo assigns every team a power rating (the NFL average is around 1500). Those ratings are then used to generate win probabilities for games, based on the difference in quality between the two teams involved, plus the location of the matchup. After the game, each team’s rating changes based on the result, in relation to how unexpected the outcome was and the winning margin. This process is repeated for every game, from kickoff in September until the Super Bowl.

For any game between two teams (A and B) with certain pregame Elo ratings, the odds of Team A winning are:

begin{equation*}Pr(A) = frac{1}{10^{frac{-Elo Diff}{400}} + 1}end{equation*}

ELODIFF is Team A’s rating minus Team B’s rating, plus or minus a home-field adjustment of 65 points, depending on who was at home. (There is no home-field adjustment for neutral-site games such as the Super Bowl1 or the NFL’s International Series.) Fun fact: If you want to compare Elo’s predictions with point spreads like the Vegas line, you can also divide ELODIFF by 25 to get the spread for the game.

Once the game is over, the pregame ratings are adjusted up (for the winning team) and down (for the loser). We do this using a combination of factors:

The K-factor. All Elo systems come with a

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