When ESPN's Sports Analytics Team rebuilt its expected points model in the offseason, one of the new features that was built in was the ability to adjust the expected points added (EPA) values based on the type of play that was called. This means we can fairly compare the results of plays against similar plays -- that is, to compare run plays to other run plays and not against passing plays. This adjustment is useful when comparing team running games to one another.
Teams almost universally run too often, which generally makes running plays have negative EPA. This is a consequence of a league-wide miscalculation and not the fault of running backs or other players who are only doing their best when called upon. Expected points also allows us to better contextualize a player's contributions compared to yards, as it accounts for the down, distance, yard line and time remaining. This means a 4-yard gain on a third-and-3 is immensely more valuable in terms of EPA than a 9-yard gain on third-and-15, despite being worth only one-third as many yards. In short, it's a measure of a team's scoring potential throughout a drive. From a running back's perspective, it is how much his team's scoring potential changes on his carries. Unlike with QBR, there is no division of credit among players.
This may be surprising, as Ezekiel Elliott of the Cowboys has outrushed the Dolphins' Jay Ajayi by 245 yards (891 to 646), but the yards Ajayi is gaining have helped the Dolphins improve their scoring potential more than Elliott's have. That isn't to say Elliott has racked up a bunch of meaningless yards, but Ajayi has been particularly efficient, evidenced by his league-leading 5.98 yards per carry. He also owns the carry on which his team gained the most EPA so far this season -- his 62-yard touchdown to salt the game away against Pittsburgh in Week 5.
Why is that run more valuable than some others that were longer? Largely because it came with only 1:01 left in the game, which meant the Steelers wouldn't have much time to respond, and enhances the value of scoring the touchdown.
For more on EPA and other NFL metrics, check out ESPN.com/Analytics