Jamaal Charles, Matt Forte, Chris Johnson, Justin Forsett, Jonathan Stewart, Danny Woodhead, Mike Tolbert, Darren McFadden, Marcel Reece, and Jerome Felton all entered the NFL in 2008. So did Steve Slaton, the rookie rushing leader that year, and Ray Rice, Rashard Mendenhall, Michael Bush, Peyton Hillis, and Felix Jones. Analyzing where the ’08 class ranks in NFL history is a project for the offseason, but today, I thought it would be fun to look at rushing yards by running backs by class year.

The graph below shows that data through six weeks of the 2015 season. As you can see, players in their 8th NFL season — those who entered the league in 2008 — are doing quite well.

The class with the most rushing yards so far in 2015 are the rookies. That class is currently led by Thomas Rawls, but has also received strong production from higher picks like Todd Gurley, Melvin Gordon, and T.J. Yeldon. After the class of ’15, there’s a gradual decline with respect to production by older classes. And then, there’s the class of 2008.

Let’s look at the data in table form, along with some other metrics.

Yr # Rush Rush Yd TD YPC 1 25 674 2713 16 4.03 2 25 683 2585 24 3.78 3 24 552 2384 20 4.32 4 24 503 1955 8 3.89 5 12 297 1087 9 3.66 6 10 338 1516 13 4.49 7 7 172 583 2 3.39 8 10 573 2446 14 4.27 9 4 170 674 4 3.96 10 3 56 241 4 4.30 11 2 117 514 4 4.39 Total 146 4135 16698 118 4.04

With respect to some of the older classes, the takeaway is more along the lines of “if you can survive in the NFL this long, you will probably be okay.” While there’s not much gross production from players in their 9th, 10th, and 11th years, that’s because there just aren’t many players from those years still around. But Frank Gore, Darren Sproles, DeAngelo Williams, Adrian Peterson, and Marshawn Lynch are still going strong.

This is in line with some interesting research by Adam Harstad, who compared running back aging to mortality tables. His view is that a more binary “yes/no” question should replace the gradual aging curves, and there’s a lot of interesting research to be done in this area.

The mortality table approach tells us that, instead of general declines, there are two types of outcomes: surviving to the next year unscathed, or falling off of a fantasy cliff entirely. In order to explain observed patterns, two things must be true about players as they age. Just like in my Voodoo hypothetical, the rate at which players “die”, (or fall suddenly and unexpectedly from fantasy relevance), must increase with age. Second, the players who do not “die” should remain relatively steady. There can be variation, of course— some will do better, some will do worse— but that variation should be fairly randomly distributed so that the population average remains constant year over year.

I’d say the results so far this year support that theory.