I’ve spent a lot of time this offseason looking at how to grade wide receivers; today I wanted to get some initial thoughts down on paper on running backs.

I’m short on time today, which means a lot of data and not so much theory. One of the more underrated statistics to measure running backs is the number of first downs they produce. I don’t like using Yards per Carry for running backs because that metric is pretty sensitive to outliers. But by using rushing first downs, perhaps we can smooth things out.

We know that the value of a touchdown is about 20 yards, but what is the value of a rushing first down? Being short on time, I took the easy way out. Pro-Football-Reference has produced Expected Points Added for each team’s running game going back to 2000. I decided to run a regression on the team level to best predict rushing EPA based on four rushing statistics. The R^2 was 0.77, but more importantly, here was the best fit formula:

EPA = -16.6 -0.58*Rush + 0.067*Rush_Yd + 1.43*Rush_TD + 1.08*Rush_FD

What interests me is the relationships between the variables. Rushing touchdowns are considered 21.3 times as valuable as rushing yards, which happens to fit in well with our previous assumptions. But more importantly, this tells us that a rushing first down is worth 16.1 rushing yards. That seems pretty high to me, and I reserve the right to adjust this later, but for now, let’s adjust down and say the value of a rushing first down is 15 yards. Now what?

There were about 12,000 rushes by running backs last season. If we give each running back 20 yards for each rushing touchdown and 15 yards for each rushing first down, then the average running back gained 8.05 Adjusted Yards per Carry last year. From there, we can — by making some broad and some inappropriate assumptions — come up with a value over average statistic.

Let’s start with Adrian Peterson. He rushed 348 times for 2,097 yards last year, and also picked up 88 rushing first downs and 12 rushing touchdowns. That gives him an average of 10.5 AY/C last year, nearly 2.5 AY/C above league average. That means over the course of his 348 carries, he is credited with 856 Adjusted Yards ((Based on the following formula: ((10.5-8.0)*348).)) above average. If we do that for every running back, we get the following table:

I think I like this better than a system based purely around yards per carry, but there are some obvious drawbacks. I don’t like that Andre Brown is ahead of Arian Foster, and in general, Foster seems way too low (you can read more about my thoughts on him here). And obviously it’s hard to separate out a player’s ability from his production, and I still like Trent Richardson even if he didn’t produce in an unfavorable situation last year.

Four years ago, I ran a series on the most dominant running backs of all time. At some point, I plan to revisit that topic. I dismissed the idea of using yards per carry to grade running backs then based on the following logic:

Is 270/1100 better than 330/1100? One argument that I’ve certainly used before is “they got the same number of yards, but the first guy’s team had an extra 60 plays with which to gain more yards!” The natural response to that is “why would a coach give a RB 330 carries if he was only getting 3.6 per carry?” From there, we have a two different answers. Either: a) the RB wasn’t that good but either the coach was dumb or the backups were really bad, or b) the RB was good and his YPC is misleading.

Once again, with rushing, I think median carry is a more telling number than average carry. Yards per carry is not a very good measure of central tendency. On the other hand, we can infer that if a RB is getting a high number of carries, he’s doing something right. Carries themselves are highly correlated with greatness.

Terrell Davis, Edgerrin James, Curtis Martin, LaDainian Tomlinson, Eric Dickerson, Clinton Portis, Eddie George, Walter Payton, Barry Sanders, Jim Brown, Emmitt Smith, Ricky Williams and Earl Campbell. Those are the RBs with over 19 carries per game for their careers. A RB that gets carry after carry is doing something right. Maybe he’s consistently getting gains, maybe he’s running hard despite a bad OL, or maybe he’s able to kill the clock without fumbling. All of those things are good. A list of the top RBs by yards per carry? Jim Brown, Barry Sanders, Paul Lowe, Robert Smith, Joe Perry, Wendell Tyler, Greg Pruitt, James Brooks, Tiki Barber, Hugh McElhenny, O.J. Simpson, Fred Taylor and Charlie Garner all have career averages over 4.6 YPC. Ignoring the overlap, I’d prefer the first list.

Remember, teams can choose to pass instead of run. So if a RB is getting 350 carries, it can’t be just because his RB teammates are bad. It’s got to be because the team’s QB is bad, too. And if a team’s QB is bad, its other RBs are bad and one guy keeps getting carry after carry, then he’s pretty valuable to his team. And if year after year he gets these carries, he’s definitely doing something right. A RB with 1600 yards on 400 carries may be just as valuable or dominant as one with 1600 yards on 325 carries. After all, the obvious question for the latter RB is — why didn’t he get more carries? Perhaps the former RB was getting three and four yards on every carry, a very valuable trait.

I’ll open up the rest of the post to you. What formula would you devise to grade running backs? Leave your thoughts in the comments.