I’ve been writing a lot about run-pass balance lately, and part of my theory of why teams are perhaps passing less often than they should has to do with the evolution of the sport. Rule changes over the recent decades have generally favored passing. Changes in pass blocking rules and in pass interference rules have made it easier to pass the ball successfully. Even subtle rule changes such as the definition of possession and “control” may have made receiver fumbles less likely.
Tactics and play selection have been refined over the years to take advantage of the rule changes, but I’m not sure that they’ve completely caught up. Results from several studies, including my own, have suggested that in most situations, passing is more lucrative than running. This imbalance implies that passing should be selected more often. As defenses respond to expect more frequent passing, the payoff for passes will decrease as the payoff for runs increases. Eventually, there is an equilibrium where the payoffs should be equal.
In this post, I’m going to look at very simple historical trends. As you’ll see in the graphs below, there is evidence that the current run-pass balance has not responded fully to recent increases in the payoff of passes. All data come from PFR's very cool league historical pages.
The first graph charts the yards per play for running and passing over the past 60 seasons. Without detailed play-by-play data, we can't rely on advanced metrics like Expected Points Added (EPA). Instead, simple efficiency will have to do. The passing efficiency I used is Adjusted Net Yards Per Attempt (Adj Net YPA). 'Net' means that sacks and sack yards lost are factored in, and 'Adjusted' means that there is a 45 yard penalty for every interception. The running efficiency I used is straight-up Yards Per Carry (YPC).
You can see the dramatic increase in passing efficiency following the 1978 rules changes. I was familiar with that increase, but what I wasn't aware of was the steep decline in passing efficiency during the '60s. As early as 1961, passing efficiency was nearly as high as it was throughout the '80s, during the Bill Walsh/West Coast Offense trend. In contrast, running efficiency has been remarkably steady, hovering around 4 YPC since the end of World War II. (I think that before then, sacks were considered rushing losses, which would explain the uptick in running efficiency in the late '40s.)
The second graph charts the number of runs and passes per game by year. We need to look at it on a per game basis because the number of games in a season has changed over the years.
You can see that the proportion of passes declines following the '60s in response to the decline in efficiency. Then there is a jump in the proportion of passes following the 1978 rules changes. It took several years for the balance to fully shift. There was a lag as coaches realized the new potency of passing.
Now look at the more recent years. Following 1984, when the increase in passing efficiency leveled off, it soon began a slow but steady climb to where it is today. But also notice there has been no corresponding increase in the proportion of passing. In 1984, passing efficiency was 5.0 Adj Net YPA, and running efficiency was 4.0 YPC, for a net passing premium of 1.0 yards per play. By 2009, passing efficiency climbed to 5.6 Adj Net YPA, while running efficiency was 4.2 YPC, for a premium of 1.4 yards per play. That's a 40% increase in the premium.
In 1984, passes represented 54% of all plays. By 2009, the proportion of passes climbed only to 56%. That's a relatively meager increase, barely above the noise of random year-to-year variation.
Here's what I think is going on: Coaches took several years to fully take advantage of the increase in passing's effectiveness following 1978, and that increase was obvious and immediate. The more recent and more subtle increase has yet to be realized. Passing, in most situations, has become more productive, but coaches haven't taken advantage of it yet.
Further, from a game theory perspective, the rock-solid steadiness of running efficiency is very surprising to say the least. If the '78 rules changes suddenly increased the effectiveness of passing, we would expect a corresponding increase in the proportion of passing, which we do see. But we'd also expect to see an increase in running efficiency as defenses adjust their schemes to counter the new realities of passing. But it's just not there.
To me, this suggests defenses have largely remained unchanged, despite the focus on pass rushing beginning with players like Lawrence Taylor. Defenses appear to stubbornly focus on the run, making certain that they keep running efficiency under control. But this focus comes at the expense of passing efficiency. Defenses are happy to let passing efficiency 'be what it will be' in their pursuit of stopping the run.
Defenses may not have a choice. Perhaps once running efficiency gets much over 4 YPC, stopping an offense becomes extremely difficult. With 3 tries to get 10 yards, perhaps 4 YPC is a magic number that the basic rules of football dictate. It could be that run defense is simply inelastic. The way modern defensive schemes are constructed, defenses are unable to shift toward stopping passes more effectively. Whatever the reason, defenses appear either unable or unwilling to adapt.
If this is true, there is little doubt that offenses should take better advantage of the current imbalance by passing more often, specifically on first and second down outside the red zone. Offenses should force defenses to respect the pass more and more until they respond, thus allowing runs to become more productive. If defenses are unable to respond, it's possible that passing in certain situations has become a 'dominant strategy.' In game theory terms, this would mean it never makes sense to run. I doubt this is the case, but if defenses are truly inelastic, that's what it would mean.