Heading into Week 6 of the 2018 NFL season, the ground game is one of the few things that is working for the Cowboys offense. Ezekiel Elliott leads the league in rushing with 480 yards, is averaging almost 100 yards per game, and is running at a 5.2 yards/attempt rate, a career-high.
Overall, the Cowboys are averaging 5.2 YPA on the ground, the second-best value in the league, and their ground game has contributed 13.56 Expected Points, the best value in the league by quite a margin.
By contrast, the passing game has generated -16.63 Expected Points, the second-worst value in the league.
But who should get the credit for the success on the ground, Elliott or the offensive line, and is such a differentiation even possible?
To see just how much the blocking is contributing to Elliott’s overall numbers, we turn to our friends from Football Outsiders. FO have developed two specific metrics to evaluate blocking and rushing performance that we’ll look at below.
Based on regression analysis, the Adjusted Line Yards (ALY) formula takes all running back carries and assigns responsibility to the offensive line based on the following percentages:
Losses: 120% value
0-4 Yards: 100% value
5-10 Yards: 50% value
11+ Yards: 0% value
These numbers are then adjusted based on down, distance, situation, opponent, and the difference in rushing average between shotgun compared to standard formations. Finally, we normalize the numbers so that the league average for Adjusted Line Yards per carry is the same as the league average for RB yards per carry [which is 4.03 yards after five weeks in 2018].
ALY penalizes an offensive line when a runner is tackled for a loss by adding 20% on top of the negative yardage (e.g. a run for -5 yards is credited with -6 yards, or 120% of the original value). It also gives progressively less credit for long runs, simply because the contribution of the line, once the runner hits the open field, quickly approaches zero.
Therefore, the first four yards of a run are fully credited, the next six yards (the running back hits the second level between 5 and 10 yards out) are only credited with half the actual yardage and for all yardage beyond the 10 yard line (the running back is now in the open field) the offensive line doesn’t get any credit at all.
Using Adjusted Line Yards (ALY), this is how the Cowboys offensive line performed so far this year, and how that compares to previous years:
|Year||Rank||Adj. Line Yards||RB Yards||NFL Avg|
|Wk 5, 2018||7||4.65||5.02||4.17|
Yes, you read that correctly: as measured by ALY, the Cowboys O-line is just as good as it was in 2016 and 2017. Sure, it slipped by three spots in the team rankings, but that’s marginal.
Over the last three years, the Cowboys’ offensive line has been clearing a path for significant chunks of yardage for Ezekiel Elliott and whoever else occasionally gets the ball in Dallas. So the question is, is the O-line making Elliott look good, or is Elliott good enough on his own, or is the answer somewhere in between?
The running backs
You cannot evaluate running backs in isolation from their O-line. But while you can’t completely separate the effect the two position groups have on each other, you can approximate it by looking at the relationship between Adjusted Line Yards and Running Back Yards, which are the standard NFL yardage numbers we’re used to.
Per the table above, the Cowboys’ running backs are gaining 5.02 yards a pop so far in 2018, the fifth-best value in the league behind the Jets (5.48), 49ers (5.26), Broncos (5.22), and Panthers (5.10). That’s pretty impressive any way you look at it.
If you deduct the Adjusted Line Yards from the Running Back Yards you’ll get a good measure of how the running backs performed relative to the line, because by taking away the contribution of the offensive line, be it good or bad, you’ll get a measure of how good a running game a team has. The table below does that calculation for the top seven teams in 2018.
|Running Back Yards minus Adjusted Line Yards, 2018|
|Rank||Team||Adj. Line Yds (NFL rank)||Running Back Yds (NFL rank)||RBY - ALY|
|1||Jets||3.79 (25th)||5.48 (1st)||1.69|
|2||Giants||2.76 (32nd)||4.10 (15th)||1.34|
|3||Chargers||4.36 (12th)||4.89 (6th)||0.53|
|4||49ers||4.79 (5th)||5.26 (2nd)||0.47|
|5||Broncos||4.82 (4th)||5.22 (3rd)||0.40|
|6||Falcons||3.53 (30th)||3.92 (21st)||0.39|
|7||Cowboys||4.65 (7th)||5.02 (5th)||0.37|
The reason these teams show up so high here is a bit of a sample size issues. Players like Saquon Barkley (4 runs of 30+ yards) or Isaiah Crowell (4 runs of 30+ yards) have broken a few really long runs in the early part of the season, thereby inflating their team’s running back yards. Those numbers are likely to come down as the season progresses.
Remember how ALY doesn’t include any yardage beyond ten yards? Well, the RB yards do, which is why teams with an over-proportional amount of breakaway runs tend to look quite good when you subtract running back yards from ALY.
The Cowboys are right up there in a second tier of teams with strong O-lines and and a positive differential between running back yards and adjusted line yards. Runners for these teams consistently get more yards than their O-line would warrant.
But Football Outsiders offer more metrics than just line yards and RB yards. Here’s a look at what their other metrics suggest about the Cowboys’ ground game.
If you hated Garrett’s decision to punt on fourth down against Houston, this is the stat for you. Here’s what power success measures:
Percentage of runs on third or fourth down, two yards or less to go, that achieved a first down or touchdown. Also includes runs on first-and-goal or second-and-goal from the two-yard line or closer.
|Year||Rank||Power Success||NFL Avg|
|Wk 5 2018||18||71%||71%|
Going by these percentages, the Cowboys would have had about a 71% chance to convert that fourth down, and that’s pretty much in line with where they’ve been the last two years. Sure, their ranking is lower, but that’s because the rest of the league has improved, not because of some sudden deficiency in the Cowboys run game.
10+ Yards or Open Field Yards
Yardage gained by running backs at least 10 yards past the line of scrimmage, divided by total running back carries.
FO are calling all yardage beyond ten yards ‘Open Field Yards’. If you take Open Field Yards divided by total carries, you get an indicator for the ‘big-play ability’, or at least the ‘breakaway ability’ of your running game.
|Open Field Yards|
|Year||Rank||10+ Yard runs / carries||NFL Avg|
|Wk 5 2018||4||1.22||0.65|
Most breakaway runs happen when a running back scoots through a hole the offensive linemen opened, often assisted by some second-level blocking. If your backs are unable to get down the field with any regularity, chances are it’s the offensive linemen that are not making it happen.
But that’s not an issue for the Cowboys, who regularly open holes for Ezekiel Elliott, which is one reason why he leads the league with 14 runs of 10+ yards.
The Cowboys running game was stuffed at or behind the line of scrimmage on 20% of all runs, which ranks the Cowboys 18th in the league. That may feel woefully inadequate for a line that ranked fourth in this category last year, but that may simply be an effect of small sample sizes at this point.
Last year’s O-line allowed a stuff rate of 17%, this year’s line is allowing 20%, a difference of just 3%. Based on the 130 rushing attempts this year, that percentage means the 2018 is off the elite pace set by the 2017 team by four stuffed runs over five games. I think they can live with that.
A lot of the credit for the Cowboys’ success on the ground must go to the offensive line and their blocking. For all the discussion about Tyron Smith’s health, Connor Williams’ alleged lack of strength, Travis Frederick’s absence, or the endless hot takes about La’el Collins’ suitability at right tackle, the O-line is clearing ground for the running back at the same pace it did in 2016 and 2017. Statistically, there is no noteworthy difference between the three units in the ground game.
But an equal amount of credit must go to Ezekiel Elliott, who produces beyond what the O-line is making possible, and producing at a better rate tan he has ever done. Taken together, this has resulted in a ground game that produces at an elite level in the league.
If only the passing game could keep up.