With leaves turning and snow falling across America, it's time for the NFL seasons to change, too.
With just two weeks to go before Thanksgiving and the regular season more than halfway over, the passing-friendly warm-weather games of September are giving way to the head-down, grind-it-out wars of November and beyond.
Last week, we looked at how this season's top NFL teams are moving the chains with aggressive power run attacks, bucking a decades-long trend toward passing.
In Week 9, for instance, 12 running backs piled up at least 100 yards rushing, per Pro-Football-Reference.com—18 of 26 teams went over 100 yards rushing. Fourteen of those teams also cracked 150 ground yards.
What's going on?
Is winter weather really turning teams toward the run game? Are defenses wearing down? Is the evolution of the NFL toward passing actually doing a U-turn? Is there an advantage to running much later in the season?
For Everything, There Is a Season
First, let's look at year-over-year changes in league-wide rushing patterns.
Using Pro-Football-Reference.com's Team Game Finder, we can find the league-wide average of per-team rushing yards and attempts from the first and last eight games. By subtracting the former from the latter, we can see if teams have been running more in the second half of the season.
Doing that for each of the last 20 full seasons, we'll see if any trends emerge:
In the early 1990s, rushing attempts and yards held steady from the beginning of the season to the end. Year by year, teams averaged a little more or fewer yards in the second half of the season, but the trend was very near zero.
From 1993 to 2012, there was a slow, tiny rise in late-season rushing attempts.
Over the last five years, NFL teams have averaged 5.68 more rushing attempts (0.71 per game) in the second half of the season than the first.
The increase in late-season rushing yardage is also small but present. Across the last five years, NFL teams have averaged 41.9 more rushing yards (5.24 per game) during the second half of the season.
Three quarters of a carry, for five more yards, per game is barely noticeable.
Over the entire league, NFL teams are running very slightly more often and very slightly more effectively in the second half of the season; this is a trend that has built up very slowly over the last 20 years.
The trends we're seeing in 2013, though, just seem to involve the teams at the top of the heap. What if we run the same comparison with only playoff teams?
Running to Win
As football researchers like Aaron Schatz of Football Outsiders have found over and over again, there's a correlation between running the ball and winning football games—but not because running a lot magically makes teams win. Instead, teams run more often when they have a lead.
They're running because they're winning, not winning because they're running.
This analysis is isolated from that effect. On the average, playoff teams win just as often in the back half of the regular season as the front. By averaging the rushing production of each season's 12 playoff teams, significant trends in rushing won't be due to the teams winning more often.
Let's see how playoff teams' late-season rushing yards and attempts trend against their early-season efforts:
Right off the bat, we see much more year-to-year variance—and much stronger trends.
Again, keep in mind: We're talking about each year's 12 playoff teams and comparing the average of all their first-half performances against the average of all their second-half performances.
In the early to mid-1990s, late-season rushing attempts of playoff teams held flat against their early-season numbers—no different than the entire league. Over the last two decades, though, playoff teams have run more and more over the second half of the season.
In the last five years, playoff teams have averaged 10.1 more second-half carries per team (1.26 per game); that's nearly twice as much as the winter bump for all 32 squads.
Playoff teams have averaged 56.7 more yards (7.09 per game) in late-season games over the last five years—again, not an earthshaking amount. However, playoff teams have averaged 125.58 rushing yards per second-half game over the last five years. That 7.09 yards-per-game figure is a 5.6 percent increase!
Interestingly, in the 1990s, playoff teams actually averaged fewer rushing yards and attempts in the second half of the season. From 1993-1997, playoff teams averaged 38.98 fewer rushing yards (4.87 per game) across their last eight games than their first eight games.
That playoff teams are now way ahead of the rest of the league in late-season running implies something else: Non-playoff teams are running less as the weather gets colder.
Flipping the Script
If the entire NFL shows a very small positive difference between early- and late-season rushing yards and playoff teams show a similar, but much larger, effect, non-playoff teams must be trending in the opposite direction.
Sure enough, they are:
In 1995, non-playoff teams averaged 81.4 more rushing yards in late-season games than early-season games, and playoff teams averaged 116.3 fewer rushing yards over the year's latter half.
In 2012, the script was flipped: Non-playoff teams averaged 49.6 fewer rushing yards across their second-half games, and playoff teams averaged 89.83 more. Somewhere along the way, the strongest teams went from leaning on the pass in the second half of the season to leaning on the run.
Clearly, the recent trend of strong teams keeping it on the ground with the leaves and snow isn't due to weather (according to my research, it still snowed in 1995). Something about the nature of the game itself has changed.
My best guess: The extreme, quickening pace of the NFL's pass-first evolution. Back in the early 1990s, the NFL was going through a mini-Dead Ball Era, a three-year period where passing and scoring were severely depressed. The teams that could pass well in critical games separated themselves.
Now, it's the other way around. Every NFL team is relying on the pass, so the teams that can run well when it counts are separating themselves.
As long as NFL offensive coordinators keep finding new ways to spread the field and throw the ball, the trend of strong teams running it down the stretch will continue.