Earlier we looked at the quarterbacks and how they differed once adjusting for opponent. It’s time to do the same for the running backs. Unlike with quarterbacks, there aren’t a dozen ways to evaluate running backs, but there should be enough for me to bloviate through another article.
Running backs and receivers are of course more dependent on their statistics from their supporting cast than quarterbacks, making them a lot more difficult to analyze from a statistical standpoint (or at least draw firm conclusions from), they’re not impossible to divine either.
Though the Vikings have been embroiled in a running back controversy of sorts, the talent disparity between the two the Vikings have left on the roster taking significant carries is not as large as many fans think, including me. There is strong evidence to indicate that though Jerick McKinnon deserves much more play from the Vikings in order to build up a stronger and easier to parse resume, Asiata has received a little too much flak from fans and analysts alike when it comes to his impact on the offense.
First, a couple of base statistics, unadjusted for the quality of opposition, then some advanced unadjusted statistics:
|Traditional RB Numbers|
|Advanced RB Numbers|
|Name||Grade||SR – FO||SR – AFA||RSR||VOA||“True” Yds||“True” YPC|
Without context, that second table doesn’t mean much. Obviously, attempts, yards, touchdowns, fumbles and first downs are easy to figure out, but after that, there’s a little bit of shorthand.
“Grade” refers to the Pro Football Focus grade of the two different running backs, something that has been subject to a little bit of debate this year because of head coach Mike Zimmer’s comments about responsibility and whatnot. In this case, “assignment” is not a big issue, because “getting as many yards as possible” is usually the responsibility of the running back when he gets the ball—though six-man protection schemes in the passing game are a little more complicated.
PFF (though better than the other statistics in this piece in this regard) does not attempt to evaluate the talent or skill of the running backs so much as the result of the play and their contribution (an implicit evaluation of skill, but still something worth taking into account).
The big difference between the two in regards to grade has to do with the passing grade, as McKinnon’s -1.6 composes almost his entire negative grade, whereas Asiata’s -2.5 is only half of his negative contribution. In the running game, McKinnon +0.4 exceed’s Asiata’s -2.7. In pass blocking, the difference is nearly negligible; Asiata’s +0.2 surpasses McKinnon’s -0.3.
McKinnon’s doesn’t mean much, however. He has only had nine snaps in pass protection and though is credited with allowing a piss-poor single sack and hurry in those nine snaps, PFF doesn’t seem to blame McKinnon entirely for them; likely a result of the pressure scheme or the time the quarterback held the ball in the pocket.
In contrast, McKinnon’s five preseason pass protection snaps were stellar, with no pressures allowed (Asiata allowed a hit and two hurries in 11 snaps) earning a pass block grade of +0.3 (Asiata: -0.8). There really isn’t much of a sample size for either, but it’s extremely low for McKinnon.
The next three statistics in the table are tagged as various forms of “SR,” which all stand for Success Rate. Success rate is a concept that has become increasingly prominent in statistics for its relationship to winning, far more than yards per carry or total rushing yards. For different third-party statistics, successes are defined differently.
Football Outsiders and I use very similar definitions (tagged as SR – FO and RSR, respectively). The RSR (run success rate) definition in the ninth column simply uses the one found in The Hidden Game of Football by Bob Carroll, Pete Palmer and John Thorn—considered one of the seminal works in advanced football statistics. For them a run is counted as a success if it achieves 40 percent of the required yardage to convert for a new set of downs on first down, 60 percent on second down and 100 percent on third and fourth down.
In that case, a run on first and ten would be considered a success if it gained four yards, not three. But on second and ten, would only be considered a success if it goes for six yards or more, no less.
Football Outsiders uses that definition for most of the game, but then modifies it based on the point differential in the fourth quarter, as the offense is defining success differently if they are ahead or behind when the game is waning (an offense who is ahead only cares about milking the clock, while an offense playing from behind needs more yards than before to have successes, especially because running consumes the clock).
For Advanced Football Analytics, a run is a success only if the play adds to the “expected points” total of a play. Brian Burke developed a model using over a decade of play-by-play data that assigns an “expected points” value for every position on the field, down and distance. When a running back runs the ball and increases the “expected points” of an offense, that’s a success.
Because any position on the field assumes a higher probability of passing the ball and running, and that the average pass gains more yards than the average run, it is difficult for a running play to add to the expected point total.
For two of the three measures of success rate (which tend to correlate highly with each other), Asiata exceeds McKinnon. Football Outsiders does not have success rate data published for McKinnon, likely because of his low carry total, but I suspect it will show much the same.
That Asiata’s success rate is higher than McKinnon’s is important. It emphasizes consistency and has a much stronger relationship to winning than yards per carry does. That said, explosiveness and yards aren’t nothing; getting 20 yards on first and ten is often better than getting 1 yard on 2nd and 1, though both are successes. Football Outsiders’ “VOA” combines the two, giving some credit for yards gained and other credit to successes, using a proprietary formula that weights them according to contribution towards winning, historically.
In this case, McKinnon’s ability to get greater yards more often outweighs his lower success rate (five percent can be a significant difference, so it’s not as if McKinnon was overcoming a small obstacle) by a significant margin. That explosiveness, unfortunately, hasn’t come in the form of a lot of high-value runs. He has one carry of 15 yards or more, which went for 55 yards. When converting all 15+ yard runs into runs credited only for 15 yards, McKinnon still ranks highly among backs with 30 or more carries, 8th overall behind players like Marshawn Lynch and DeMarco Murray. Among that same group, Asiata ranks 24th.
Still, Asiata has been more successful more often, even when separating by down.
|Name||15+ Runs||15+ Yards||Muted YPC|
|Name||YPC 1st Down||YPC 2nd Down||YPC 3rd Down||SR 1st Down||SR 2nd Down||SR 3rd Down|
McKinnon’s increased success rate on third down is certainly interesting, though almost entirely because McKinnon has only had three third-down carries. Five yards on 3rd and 11 against the Falcons, nine yards on 3rd and 7 against the Falcons and 10 yards on 3rd and 1… against the Falcons.
This in part explains Asiata’s abysmal VOA score. Asiata hasn’t had any big gains and despite his relative ability to create successes, doesn’t stack up against his peers on a down-by-down basis, and creates fewer first downs per carry because his carries stop at “success” without gaining that much more.
I did skip over one set of stats—”true” yards and “true” yards per carry, something I roughly derived from Chase Stuart’s work on “true receiving yards,” which is work designed to adjust receiving yards for opportunity and value added through first downs and touchdowns.
Instead of using attempts instead of receptions or first downs, I used successes—each success was counted as 1.67 yards, each touchdown as ten yards (instead of the 20 they get in the passing game to account for the relative fungibility of rushing touchdowns against passing touchdowns and double counting successes), with each fumble counting as negative 45 yards.
After that, they are normalized for team rushing attempts and yards, so that everything looks like rushing yards would this point throughout the season if the team had attempted an average number of attempts (and prorating some Adrian Peterson runs onto the two of them).
Asiata’s high success rate and good touchdown rate have improved his yards per carry, with a massive fumble penalty applied to him (relative to total number of attempts). McKinnon’s solid average was boosted relative to his peers with his low fumble rate, despite a lack of touchdowns.
This ends up having an opposite effect than VOA, interestingly, in that it emphasizes success more than FO seems to. Asiata still ends up comparing less than favorably to McKinnon, but not to nearly the same extent.
One great thing about these numbers is that they can be adjusted to opponent to get a “truer” understanding. The Vikings have been playing opponents who generally have not been great against the run, so the opponent-adjusted numbers are lower than the non-adjusted numbers.
|Opponent-Adjusted Traditional RB Numbers|
Neither McKinnon nor Asiata could score touchdowns at a rate commensurate with the strength of their opponents, save for Asiata’s three touchdowns against Atlanta—which were almost made up for by Atlanta’s relative inability to stop rushing touchdowns and a lack of touchdowns on other carries. McKinnon’s negative touchdown rate is an indication that against the opponents he played, an average RB would have scored more touchdowns despite only 30 carries.
That creates these statistics that I was able to adjust for opponent, or in the case of VOA, found an adjustment for:
|Opponent Adjusted Advanced RB Stats|
|Name||Att||Grade||YPC||RSR||DVOA||“True” Yds||“True” YPC|
Asiata had a greater number of carries against weak running defenses, so he gets penalized more per run than McKinnon does. Those adjusted statistics are truly awful for Asiata—and it puts his run success rate into a lot of context.
I’ve waffled back and forth on the value Asiata provides in light of the high value of success rate—boom-bust RBs with high yards per carry but low success rate. Though these statistics likely don’t mean much without around 150 carries for real stability, adjusting for the fact that Asiata’s best days were against defenses that had a tendency to give running backs their best days might mean that the Vikings’ entirely reasonable respect for Asiata’s ability to get “just enough” might be a product of opponent more than his ability.
Without a reasonable sample, it’s not enough to say with a strong degree of confidence that McKinnon is the superior back. But so far, we have some indication that McKinnon deserves more chances to prove it than Asiata.