I looked at all players (excluding quarterbacks) since 1990 with at least 50 carries in a season. I then grouped those players into 50-carry increments based on their number of carries that season: i.e., 50-99 carries, 100-149 carries, and so on. The chart below shows that data, along with the average number of carries for each group, the average number of rushing yards, and the average yards gained per carry:
| Carries | # of Players | Avg Rush | Avg Rush Yard | Avg YPC |
|---|---|---|---|---|
| 50-99 | 619 | 72 | 288 | 4.03 |
| 100-149 | 338 | 122 | 491 | 4.01 |
| 150-199 | 277 | 172 | 689 | 4.00 |
| 200-249 | 222 | 223 | 927 | 4.15 |
| 250-299 | 181 | 272 | 1147 | 4.21 |
| 300-349 | 127 | 322 | 1361 | 4.22 |
| 350+ | 39 | 371 | 1642 | 4.42 |
Those results are probably not very surprising. The players with the most carries have the most rushing yards, and the yards per carry average tends to increase, too. This is in some ways an example of survivorship bias: the players who are performing the best will continue to keep getting carries, moving them into the higher-carry buckets.
Now, what happens the next season? Take a second and think about what you expect….
How many rushing yards, on average, will the 200-249 group have?
What about the 300-349?
What about yards per carry?
And why do you think the way you do?
Now, let’s look at the results…
| Carries | # of Players | Avg Rush | Avg Rush Yard | Avg YPC | N+1 Rsh | N+1 RshYd | N+1 YPC |
|---|---|---|---|---|---|---|---|
| 50-99 | 619 | 72 | 288 | 4.03 | 61 | 249 | 4.08 |
| 100-149 | 338 | 122 | 491 | 4.01 | 91 | 373 | 4.09 |
| 150-199 | 277 | 172 | 689 | 4.00 | 128 | 517 | 4.04 |
| 200-249 | 222 | 223 | 927 | 4.15 | 154 | 640 | 4.15 |
| 250-299 | 181 | 272 | 1147 | 4.21 | 209 | 851 | 4.07 |
| 300-349 | 127 | 322 | 1361 | 4.22 | 262 | 1095 | 4.18 |
| 350+ | 39 | 371 | 1642 | 4.42 | 263 | 1099 | 4.18 |
And if we include games played data…
| Carries | # of Players | Avg Rush | Avg Rush Yard | Avg YPC | N+1 Rsh | N+1 RshYd | N+1 YPC | Yr N G | Yr N+1 G |
|---|---|---|---|---|---|---|---|---|---|
| 50-99 | 619 | 72 | 288 | 4.03 | 61 | 249 | 4.08 | 12.9 | 10.3 |
| 100-149 | 338 | 122 | 491 | 4.01 | 91 | 373 | 4.09 | 13.5 | 10.6 |
| 150-199 | 277 | 172 | 689 | 4 | 128 | 517 | 4.04 | 14.2 | 11.9 |
| 200-249 | 222 | 223 | 927 | 4.15 | 154 | 640 | 4.15 | 14.6 | 11.8 |
| 250-299 | 181 | 272 | 1147 | 4.21 | 209 | 851 | 4.07 | 15.2 | 13 |
| 300-349 | 127 | 322 | 1361 | 4.22 | 262 | 1095 | 4.18 | 15.8 | 13.8 |
| 350+ | 39 | 371 | 1642 | 4.42 | 263 | 1099 | 4.18 | 15.9 | 12.9 |
So, on average, the players with the most carries one year have the most rushing yards the next year. That’s probably not too surprising, and it’s why it makes it difficult to analyze whether running backs are harmed by having too many carries in a single season. After all, the players with the most carries one year do the best the next year, but that’s because there’s a third variable that is heavily correlated with the first two variables: talent.
It’s easy to see a player like DeMarco Murray have a ridiculous season like he did in 2014 (392 carries, 1,845 yards), see his production drop off the next year, and then assume the two factors are correlated. But correlation doesn’t equal causation, and we don’t know if Murray’s 2015-2017 would have looked any better if he had say, 292 carries in 2014.
What sort of studies would you be interested in seeing done on rushing attempts and future production?