The NFL leader in expected points added for a running back? It's not rushing leader Adrian Petrson. It's not Marshawn Lynch, Doug Martin, Alfred Morris or Jamaal Charles -- the rest of the top five in rushing yards.
With 40.2 EPA -- a full 17 points clear of second-place Peterson -- Spiller holds the lead through Week 14. Despite only 144 carries -- 25th in the league, between Vick Ballard and Jonathan Dwyer -- he ranks 12th in rushing yards. His 944 on the season puts him between BenJarvus Green-Ellis and Shonn Greene, rushers with 94 and 86 more carries respectively.
Now, with Fred Jackson on injured reserve with a right knee injury, Spiller will be alone at the forefront of the Bills offense. Of course, Bills coach Chan Gailey qualified his statement:
"We'll play CJ as much as he can play," Gailey said. "Tashard [Choice] will rest him when he needs rest."
Last time Spiller was healthy and Jackson was out -- Week 10 against Miami -- Spiller received 22 carries, a career high, rushing for 91 yards and adding three receptions for 39 yards. At -0.2 EPA (-0.01 per play), it was his third-worst game of the season, but still better than the league running back average of -0.02 EPA per play.
Still, when a player puts up numbers like Spiller's over the past two seasons -- 6.0 yards per carry on 251 rushes -- one would figure his coaches have a reason for limiting his carries so heavily -- and a better one than just Fred Jackson's roster presence, capable as he may be.
But so far in his career, the only two times Spiller has gone over 16 carries -- the aforementioned Week 10 against Miami and 2011 Week 11 against the Jets (19 carries, 55 yards) -- he has had trouble. It's easy to get caught up in efficiency, but in many cases volume is valuable in and of itself.
So, the question: is there reason to believe running backs get worse with more carries? In sports, we often see a usage-efficiency tradeoff -- pitchers get worse the deeper they go in games, basketball players tend to shoot a worse percentage the more looks they take from the field, and, as detailed last week, quarterbacks get less efficient the more than have to throw the ball. Does the same apply to runners?
To answer the question, I pooled all games of at least five carries and fewer than two passes (to weed out running quarterbacks) from 2010 through this season, a total of 2349 games featuring rushers from Aaron Brown to Willis McGahee.
There actually appears to be a positive correlation -- more carries means more yards per carry -- but this is lkely due to survivor bias. That is the players who don't earn at least 10 carries don't get them, dragging the average down at the low end, and the players who get 30+ carries are typically destroying opposing defenses. Chopping off the extremes and looking at games with 10-30 carries gives us a nearly flat relationship:
According to this relationship, an 20-carry increase is associated with a 0.3 YPC increase -- hardly anything that would turn a great running back into a replacement-level player or vice-versa. Bias is probably still at play at some level, though -- whether due to player skill or due to rushers getting more carries against bad defenses. Let's turn the focus to some individual players. In the sample, there are 29 players with at least 30 games of five rushes or more. Here they are shown individually:
The physical limit argument may actually work in the rusher's favor. Although backs are taking a beating every time they plunge through the line, they dish one out as well, and the defenders battle in the trenches (as they say) on the passing plays as well. We hear all about the backs who improve as the game goes on -- their endurance can outlast the defense's.
Despite his impressive accomplishments throughout his first three seasons, C.J. Spiller has yet to show his stuff throughout a 20-30 carry game. Fred Jackson has limited his opportunities. But the Bills' coaches reluctance to run Spiller over and over again suggests a lack of confidence in his ability to handle the workload. Although the Bills coaching and training staff should know his body and capabilities far better than the observer at home (i.e., me) they can't truly find out what he can do with a larger load until they allow him to try. Spiller handled well over 20 touches per game his senior year with Clemson (he also returned punts and kicks) and rushed for a better average than in either his sophomore or junior years.
There's little statistical reason to believe a significant decline is in order either, and Spiller starts at such a lofty point where even the dropoffs exhibited by Pierre Thomas and Donald Brown with heavy carry loads would still leave Spiller well above average. It looks the Bills will finally free Spiller as this season closes, beginning this Sunday in Seattle. If Spiller can even come close to maintaining what he's done with limited loads so far, we could be in for some great performances over the next three games.