The 3-4 defense appears to be the wave of the future. Over the course of the recent decade, the number of teams that have employed the 3-4 defenses went from 3 to 14. This season, almost half the league will be fielding a 3-4 to one degree or another.
As the pass has become more important, it makes sense that defenses are adapting by replacing larger, slower players with faster, more agile ones. The unpredictability of the 3-4 front makes the offensive line's job that much harder. The success of defenses such as the Steelers, Ravens, Jets, and even now the Saints, suggests there may be a substantial advantage to the 3-4.
Using the concepts of Expected Points Added (EPA) and Win Probability Added (WPA), we can compare the effectiveness of the two defensive schemes over the past 10 seasons. EPA measures every play in terms of the change in the potential gain of net point advantage. WPA measures every play in terms of how much it changed a team's chances of winning the game at hand.
Over the past 10 seasons, here is how the two schemes compare in terms of EPA, WPA, and yards allowed. (Negative numbers are good. Spikes, kneel downs, and aborted plays were excluded.)
Advantage of 3-4 In:
EPA vs. Pass
EPA vs. Run
WPA vs. Pass
WPA vs. Run
Gain vs. Pass
Gain vs. Run
Defenses employing the 3-4 have allowed 1.2 fewer EPA per game than 4-3 defenses. They have given their teams an additional 4% greater chance of winning each game, yielding 11 fewer yards per game than their 4-3 counterparts. The 3-4's advantage is sizable against both the run and the pass.
This does not mean that 3-4 defenses allow an average of 1 less point per game. Rather, they create one more point worth of net point advantage, in terms of points both scored and allowed, because defenses can create turnovers and other events that lead to scores. One point may not seem like much, but in terms of win probability, it's like taking an average .500 team and turning it into a .540 team. That's substantial.
It’s still not clear exactly how 3-4 defenses are able to achieve better success. 3-4 defenses featured a higher sack rate than 4-3 defenses, 6.6 % compared to 6.3%. But the 4-3 had a higher interception rate, 3.0% compared to 2.9%. I'll leave the question to the Xs and Os guys to debate.
Note: Because both offensive effectiveness and the number of 3-4 defenses have been increasing over the past decade, I had to account for year. To do this, I first compared 4-3 and 3-4 defenses within each season, then averaged those comparisons.
Ultimately, it may be that 3-4 defenses may simply have had better talent over the past decade, which would create the appearance that the 3-4 is somehow superior. In part 2, I'll account for individual team strength to get a better estimate of the true advantage of the 3-4 in the modern NFL.