In the comments to the Greatest Running Back of All-Time Post — and reminder, entries are due by midnight Thursday — a debate broke out between sn0mm1s and Jay Beck , among others, about how to value running backs generally, and specifically, the value of long runs.

One idea I’ve had before is that the yards a player gains after picking up a first down are similar to the yards picked up by a returner. For example, when a punt returner gains 10 yards instead of 5, that’s obviously worth 5 additional yards of field position to his team. But it’s not as valuable as 5 yards on 3rd-and-5; the return yards were gained outside of the context of the down-and-distance/series-of-downs nature of the game.

Does this mean that all yards gained after a first down are exactly as valuable as return yards? I’ll leave up that to the reader to decide. But I do think one thing is noncontroversial: Lamar Miller ran for a 97-yard touchdown on 1st-and-10 against the Jets in week 17, the most valuable 10 yards during that run were the first ten. The last 87 yards were slightly less valuable (on a per-yard basis), or akin to the yards a player would gain on a return.

At this point, you might be thinking, “Who cares?” And that’s a very good question: after all, return yards are valuable. And the last 87 yards of Miller’s run were certainly more valuable to Miami than the first 10 yards, even if that may not be true on a per-yard basis.

But I thought it would be interesting to look at all running plays this season, and break them into two categories: yards that came after a first down had already been achieved, and all other rushing yards. So a 10-yard run on 3rd-and-5 has five yards in each bucket; if it was 3rd-and-1, 9 yards get assigned to the “excess yards” bucket, and 1 yard to the “going towards picking up a first down” bucket.

Where this is most interesting is at the extremes, and no player was more extreme than Justin Forsett. The Ravens back led the NFL with 17 runs of 20+ yards, just one of the drivers of his great season. But 39% of the 1,267 rushing yards gained by Forsett in ’14 came after he had already picked up a first down.

Now, in the abstract, that might mean nothing at all to you. So let’s look at some other players. You will not be surprised to learn that only 14% of Trent Richardson’s yards came after he had already picked up a first down. But Matt Forte had a great year for the Bears, and only 21.5% of his rushing yards came after gaining a first down. What’s that mean? Well, as usual, it’s complicated. First, let’s present the data.

This year, 27.1% of all rushing yards came after picking up a first down. Therefore, given that Forsett rushed for 1,267 yards, we would expect him to have gained 343 yards after gaining first downs and 924 yards prior to gaining a first down. Since Forsett actually gained 491 Excess Yards — i.e., rushing yards gained after picking up the first down — he is credited with 148 excess yards over average.

Here’s how to read the table below. Forsett rushed 235 times for 1,267 yards, and averaged 5.39 YPC. He picked up 491 excess yards, which means 38.8% of his yards came after gaining the first down. He also picked up 147.7 Excess Yards over Averaged, the most in the NFL last year.

Excess Yards are not intrinsically a bad thing. One of the fun parts of analytics is moving away from the Good/Bad debate, and just getting a broader and deeper sense of understanding of the game. Bishop Sankey and Matt Asiata are at the bottom of this list, and that makes a lot of sense, too.

That said, I think there may be something to be done here. So I open it up to you: what do you think of this concept?