Stats Explained

By Michael Gertz
Thursday, July 5, 2012

Our stats at ProFootballLogic are all in terms of "EPA", or expected points added. Every single play is assigned an EPA value based on the difference in expected points for the drive from the beginning of the play to the end. Expected points are tabulated based only on the current down, distance to go, and field position. They are a league average and therefore do not vary from one team to another. Here are some examples of expected points:

(A) 1st and goal, at opponent 1 yard line: 5.58 expected points
(B) 1st and goal, at opponent 10 yard line: 4.16 expected points
(C) 4th and goal, at opponent 10 yard line: 2.17 expected points
(D) 2nd and 5, at opponent 45 yard line: 1.92 expected points
(E) 1st and 10, at 50 yard line: 1.82 expected points
(F) 2nd and 10, at 50 yard line: 1.39 expected points
(G) 3rd and 10, at 50 yard line: 0.72 expected points
(H) About to receive kickoff: 0.70 expected points
(I) 1st and 10, at own 14 yard line: 0.01 expected points
(J) 1st and 10, at own 1 yard line: -0.65 expected points
(K) 4th and 10, at own 1 yard line: -2.22 expected points

These vales essentially represent an average along the lines of "(Odds next score is offensive TD)*(7)-(Odds next score is field goal by defensive team)*(-3)...etc". The exact values vary slightly from this because the value of receiving a kickoff is slightly better than neutral. Because of this, scoring a touchdown is actually worth slightly less than 7 points and a field goal is slightly less than 3 points in the long run. Likewise, situations where those results are very likely will be worth slightly less than 7 and 3 expected points respectively. We can see that the break-even point for the start of a drive is about a team's own 14 yard line.

To arrive at expected points added for any play, we simply take change in points and expected points that occured duing the play. A positive value can be considered a successful play, and a negative value can be considered an unsuccessful play, compared to league average performance. EPA values are the same for the same play outcome regardless of what happened during the play. If the defense has the ball at the end of the play, their expected points at the end of the play can be considered as negative expected points toward the offensive EPA. Here are some examples of EPA using the above expected points references:

1st and 10, at 50 yard line -- 5 yard pass completion: 0.10 EPA (D-E)
1st and 10, at 50 yard line -- 5 yard run: 0.10 EPA (D-E)
1st and 10, at 50 yard line -- incomplete pass: -0.43 EPA (F-E)
1st and 10, at own 14 yard line -- 86 yard TD: 6.29 EPA (7-H-I)
1st and 10, at own 14 yard line -- 72 yard pass intercepted, no return: 0.00 EPA (I-I)
1st and 10, at own 14 yard line -- interception returned for TD: -6.31 EPA (-7+H-I)
1st and goal, at opponent 1 yard line -- interception returned for TD: -11.88 EPA (-7+H-A)
4th and 10, at own 1 yard line -- 49 yard punt out of bounds: 0.40 EPA (-E-K)

EPA for a drive or a game is simply the sum of EPA for those plays. The difference in offensive EPA between the 2 teams in a game will equal the score difference (apart from a few strange play types our system does not account for to keep the ratings more meaningful). Some general guidelines are that it takes about 20 yards gained to add 1 expected point with equal down/distance, and the difference between 1st and 10 and 4th and 10 is about 2 points lost at equal field position. This means that in general a 40 yard drive and then punting can be considered a neutral result.

Team stats values on this website are in terms of EPA per game. Defensive values are EPA allowed, so a smaller or negative number is better.