Why The NFL Can’t Rely On Defense
In an NFL season marked by historic offensive production and a championship round that was conspicuously absent a top-10 defense,2 aficionados of low-scoring rock fights, filled with punts and field goals, have been left disappointed. The best defensive teams to make the playoffs were eliminated early in the tournament, with the Bears, Ravens and Texans all losing in the wild-card round. A week later, Joey Bosa and the emerging Chargers defense were dismantled by the Patriots, and the Cowboys — perhaps the best defensive team left in the divisional round based on their end-of-season play — lost to the Rams. Extracting the strong defensive teams with relatively weak offenses led to historically exciting playoff football, producing two overtime games in the championship round for the first time in NFL history. Now we have a Patriots and Rams Super Bowl pitting perhaps the greatest QB of all time in Tom Brady against the hottest young offensive mind in the league in Sean McVay.
We shouldn’t be surprised that great offensive teams have made it this far. Teams are more reliably good — and bad — from game to game and year to year on offense than on defense. Individual defenders often have wild swings in performance from season to season, and defensive units forecast to be dominant often end up being merely average. The Jacksonville Jaguars’ defense took them as far as the AFC championship a year ago, but that same defense led them to five wins this season. Meanwhile, performance on offense is generally easier to forecast, making investments on that side of the ball more reliable.
Even then, football is largely unpredictable. When an otherwise sure-handed Alshon Jeffery3 lets a well-thrown Nick Foles pass sail through his fingers for an interception to end the Eagles season, or when Cody Parkey double-doinks a partially blocked field goal to end the Bears’ playoff hopes, we are essentially cheering, or bemoaning, randomness. Most vexing for forecasters and league observers trying to make sense of things is that the plays that matter the most in football are often the most unpredictable. But again, this is particularly true on the defensive side of the ball.
Turnover margin is the canonical example. Teams that win the turnover battle go on to win their games at a very high rate. Home teams win about 73 percent of their games when they are plus-1 in turnover differential, according to data from ESPN’s Stats & Information Group, and the home team win rate climbs to more than 86 percent when it’s plus-2 or better.
Yet despite their clear importance, the number of turnovers a team creates in one season has no bearing on how many turnovers the team will create in the next. Both interceptions and fumbles are completely unpredictable from season to season at the team level. And this pattern holds true for defense in general. If we measure the stability of defensive stats from one year to the next,4 we find that compared with offensive performance, most defensive stats are highly variable from year to year.
Defensive performance is unpredictable
Share of performance across various team-level metrics predicted by the previous season’s performance in the regular season, 2009-2018
Total offensive DVOA
Offensive passing DVOA
Defensive passing DVOA
Offensive rushing DVOA
Total defensive DVOA
Defensive rushing DVOA
Source: Football Outsiders
High-impact plays on defense turn out to be the least predictable. And while we’re by no means great at identifying which teams will succeed on offense, offensive DVOA is about twice as good at forecasting future performance as defensive DVOA.5
For teams like the Chicago Bears, who won 12 games despite fielding the 20th best offense in the NFL, this has major ramifications. The Bears were third in the league in turnover margin and third in sacks — feats we shouldn’t expect to repeat based solely on this season’s results. (Just ask the Jags.) Casting even more doubt on their ability to field an elite defense in back-to-back years, Chicago also lost its defensive coordinator, Vic Fangio, who left to become the head coach in Denver, further destabilizing the strength of the team.
Still there is some hope for lovers of the three-and-out. While rare, there are plays a defense makes that do tend to carry over from year to year. One of the most stable defensive stats is hits on the quarterback, which has a relatively impressive year-to-year r-squared of 0.21 — better even than total offensive DVOA, which is the gold standard for stability in team metrics. Quarterback hits include sacks — 43.5 percent of QB hits end in a sack, and those by themselves tend to not be predictive — but also plays in which the passer is contacted after the pass is thrown, and that contact is incredibly disruptive to a passing offense.
When a quarterback is hit, his completion percentage is affected on a throw to any part of the field.6 Teams that can generate pressure that ends with contact on the opposing QB greatly improve their chances of causing incompletions and getting off the field. And best of all, teams that are good at generating hits on the quarterback tend to stay good at it.
Philadelphia led the league in QB hits but not sacks
Total quarterback hits, sacks and expected sacks for teams’ defensive lines in the regular season, 2018
Show more rows
Sources: NFL, Elias Sports Bureau
The Eagles, Jets and the Seahawks all appear to have better days ahead of them on defense. Each team racked up more than 100 QB hits in 2018. But they also experienced bad fortune, converting their hits into sacks at a rate below what we’d expect. If these teams generate similar pressure next season, we shouldn’t be surprised to see their sack totals rise just based on reversion to the mean. Meanwhile, Chicago, New Orleans and Kansas City experienced good fortune in 2018, converting their QB hits at a rate higher than we’d expect. Assuming the defensive lines return largely intact, we probably shouldn’t be surprised to see their sack totals dip next season.
Stats like QB hits are rare to find on defense. And because of the high variance in defensive performance, teams built with a defense-first mindset end up controlling their own destinies less than we might expect. When it comes to team-building, this suggests that investments on offense are better long-term bets for stability. The results this year are particularly encouraging. Lighting up scoreboards by focusing on scoring points instead of preventing them has proved to be both successful and incredibly entertaining to watch. For this season at least, defense isn’t winning anyone a championship.
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