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Discussion in 'On The Field' started by Statalyzer, May 13, 2021.
Two guys stand out the most - both Longhorns, both KC Chiefs too.
Eddie George not looking too good. Ezekiel Elliott way better than Emmitt Smith—did the span of years they use omit all of Smith’s good years?
but to your point, Jamaal and Priest rock!
So only 5 guys had a positive expectation that giving them carries would actually add to your score over time? That seems odd.
Emmit peaked in the early to mid 90s. This info is from 2000 onward.
Great find, Stat!
I think they way they do that is by looking over what actually happened in thousands of games in the past, and asking "in this situation, which team scores next - the one currently on offense or the one currently on defense - what percentage of the time, and how many points?"
For a simple example with way too small a sample size that I'll pull out of the air - let's say they have data on ten games where a team had the ball at their own 47 yard facing 2nd and 6. Six of those times, the next score in the game came by the team with the ball (not necessarily on that same possession) - 3 TD and 3 FG = +30 pts. Four times, the next score in the game came by the team currently on defense (not meaning a defensive score, probably they got the ball back and then scored), 2 TD and 2 FG = -20 points. 30 - 20 = 10. 10/10 = 1. So a team with 2nd and 6 at their own 47 would have an EPA of +1.
Now let's say the Chiefs have 2nd and 6 at their own 47, and Jamaal runs for 10 yards. Now they have 1st and 10 at the opponent's 43. If EPA for that yardline, down, and distance is +1.4, then Charles gets credit for adding 0.4 expected points with his run. Next play he runs for 3 yards and it's 2nd and 7 at the opposing 40; if this EPA is +1.3, then Charles gets debited 0.1 expected points, and now has 2 carries for 0.3 EPA, or 0.15 EPA/carry.
So, while it surprises me that so few guys are positive, it makes sense that your average runner, who gets a lot of 1-3 yard carries and then breaks a 20 yarder a couple of times, would have an EPA slightly in the negative. It fits with the idea that running is more of low-risk low-reward and passing is high-risk high-reward, since pass plays have a higher EPA overall, but also have a lot more large negatives than runs do, since there are more turnovers and since sacks tend to lose bigger yards than a negative run play.
Your description is correct. A similar stat is WPA (win probability added) which compares a team's win probability before and after each play. Here's a good source discussing both of these stats.
Optimum Pass/Run Ratio in NFL (davidschmerfeld.github.io)
The fact that so few runners have positive EPA/WPA results from the fact that NFL teams runs too often. At the moment of the snap, the average passing play starts off with a positive EPA/WPA, while the average running play starts off with a negative EPA/WPA. Not surprisingly, only the very best runners are able to overcome this statistical disadvantage.
More surprising to me is that passing plays do better than running plays in almost every game situation. Even on 3rd and 1 or less (as well as 4th and 1 or less), passing plays have a slightly better EPA/WPA than running plays. This is not to say that NFL teams should always run in these situations -- just that they should run more often, which would tend to bring the run/pass results into equilibrium.