Powered by Defense

Offense wins in the NFL. That isn’t new information, and we don’t even have to go back farther than last season to find evidence of this. Last year the final four teams alive in the playoffs ranked numbers one, two, four, and five in offensive DVOA. New Orleans was the highest ranked defensively at number 11, and both the Rams and the Chiefs were in the bottom half of the league. If you want to win in the NFL, you do it by scoring the football, not by keeping the opposition off the board.

Not only is offensive success more useful for winning in the immediate future, it is far easier to sustain over a long period of time. As I found a few years ago, offensive DVOA is about three times as consistent year-to-year as defensive DVOA. A defense can be great one year, and mediocre the next, while offensive dominance is usually here to stay.

There is another side of this however. For teams that didn’t have success a year ago, the defensive side of the ball is often much easier to turn around than the offense. And we are seeing clear evidence of this early on this year, as several of the most surprising teams in the league have been powered by a leap forward on defense.

Before we get to them, however, we should address one team that is clear evidence of the trickiness of defensive evaluation. Because while the Buffalo Bills have been great on defense this year, riding that side of the ball to a surprise 4-1 record, they were actually even better a year ago. They finished 2018 second to only Chicago in defensive DVOA, and it still only earned them a 6-10 record. They failed because they were dead last in special teams DVOA and only better than one team on offense, and this year those two units have leapt up to…31st and 27th respectively. This is the exact same team they were a year ago, and their better performance is an indication of just how spurious defensive powered victories can be.

This is a warning for the rest of these teams that have ridden their defenses to hot starts of the season. There is one team however that doesn’t have to worry themselves about that—Buffalo’s division rivals in New England. The Patriots are going to keep winning games, because that’s what the Patriots do. But so far this year their victories have taken on a strange twist, powered by the league’s best defense rather than their future Hall of Fame quarterback.

For New England defense has always been a luxury rather than a necessity. Year in and year out they win games more consistently than any other team, as their defense oscillates between good and downright terrible. But for the first time since the early years of Brady’s career, they have a genuinely great defense, right as their offense has finally started to sputter.

The formulas used in Buffalo and New England are both very similar. These two defensive backfields boast as much talent as any others in the league, and their schemes rely on dominance on the back end to stifle the offense. They play very aggressive man coverage, sticking to receivers off the line and cutting down throwing windows to virtually nothing. The strength on the back end with stars like Stephon Gilmore and Tre’Davious White allows them to be aggressive with blitzes along the front, leaving quarterbacks harried with very little idea where they are supposed to go with the ball.

It’s fitting that one of the surprises of the NFC fits into this same mold, unsurprising with a head coach that comes from the Patriots tree. Matt Patricia’s rookie season in Detroit was a disaster, but his second year is off to a solid start, a 2-1-1 record on the strength of a defense that has jumped from bottom five in DVOA a year ago to 11th so far in 2019.

The changes in Detroit haven’t been as much about scheme as they’ve been about talent. They made a big free agent acquisition in the offseason when they gave $90 million to Trey Flowers, and while he’s only produced a single sack through four games, that was never the biggest part of his game. He does all the little things as an edge rusher and a run defender to make Patricia’s scheme function, and it has opened things up for the rest of the unit to make plays. Key among them is another offseason addition Justin Coleman, who has played out of his mind through the early part of the season and kept the Lions secondary afloat even as star cornerback Darius Slay has missed time.

We only have to go across the division once again to see another team that has leapt forward due to an influx of talent, though in the case of the Packers the talent came through the draft rather than free agency. Though they’ve certainly benefited from some veteran talent as well, with Za’Darius Smith and Preston Smith making up one of the league’s best pass rushing duos early in the season.

The real story in Green Bay is that years of investing high draft picks on the defensive side of the ball have finally started to pay off. They haven’t used a first round pick on offense since 2011, and in both 2017 and 2018 they used each of their first two selections on defensive backs. Of course, no franchise dedicates that many resources to one part of the team unless their earlier selections didn’t work out, but given enough volume they were bound to find some success eventually.

“Some success” might be understating it though. Between the highly drafted Kenny Clark and the late round find of Dean Lowry, they rebuilt their defensive line in the 2016 draft into a unit that has slowly coalesced into one of the most intimidating in the league. Throw in occasional contributions from rookie Rashan Gary, and there is a lot of pure athleticism up front to torment offensive lines.

Of course, Gary was only one of two first round selections this season, and so far he has been the much quieter of the two. Their second selection, safety Darnell Savage, has already become an integral part of their defense. He still has some work to do in terms of positioning and tackling in space, but his athleticism and aggression has already produced a number of big plays, and he has the look of a long term star generating turnovers at the back of Green Bay’s secondary for years to come.

That seems to be the general philosophy of the Packers defense this year. They’re willing to give up some chunks of yards, because they trust that they’re going to make a play sooner or later. They play an extremely pass-slanted scheme, leaving light boxes and challenging opposing teams to consistently run the ball down the field. If a team is willing to stick to the run, they trust their front to wreak enough havoc to beat blockers and knock them off schedule. And if the opposing team tries to pass, they have a hungry defensive backfield ready to make plays.

And this is where I have to talk about Jaire Alexander, because he is quickly establishing himself as one of the best cornerbacks in the league. In his second year he has already earned the trust of his coaches to be left on an island on the outside, and while he’s still liable to be beaten some (the Vikings homer in me has to point out that Stefon Diggs eats him alive, even if Kirk Cousins refuses to look his way) he is getting better every week and is likely only a little more development from becoming a true shutdown cornerback.

There’s been some debate over the past few years about how to best build a defense. The traditional philosophy was to build from the front back, believing that pass rushers have a greater impact than defensive backs. After all, even a true shutdown cornerback can only erase one of the five eligible receivers. A good pass rush can shut down any hope the quarterback has of completing the ball.

The tricky part is that as defenses have been built around the pass rush, offenses have shifted in ways to take away that impact. Quarterbacks get the ball out of their hand quicker than ever before, often fast enough that even a totally unblocked pass rusher can’t do anything to affect the play. And so defenses have been forced to adapt as well, finding cornerbacks who can erase the early opportunities while still not being beat on longer developing plays.

This is the approach taken by the four teams I’ve identified above. Because while there are good pass rushers on each of these teams, there are no real game wrecking stars along the front like Aaron Donald or Khalil Mack. The strengths of these defenses are players like Gilmore, White, Slay, and Alexander, cornerbacks who can be trusted to match up man to man with an opposing wide receiver and challenge him from the moment he crosses the line of scrimmage.

This is a clear trend among many of the defenses that have outperformed expectations this season, but that doesn’t mean there aren’t exceptions. And to find one such example, we need to turn to the team that is likely the biggest surprise of the early season. Joining New England as the last remaining undefeated team, the San Francisco 49ers have ridden the second ranked defense in the league to this point on the strength of a defensive front loaded with more highly drafted talent than any other unit in the league.

Entering the offseason the 49ers had spent three of their past four top selections on the defensive line. It was a mixed bag, from a genuine star (DeForest Buckner) to an unspectacular starter (Arik Armstead) to a major disappointment (Solomon Thomas). But it was the one part of their roster they had invested most heavily in, which made it slightly surprising when their biggest addition in both free agency and the draft were pass rushers.

But so far in 2019 the additions of Dee Ford and Nick Bosa have both paid off in a major way. Ford hasn’t quite lived up to the 13 sacks and seven forced fumbles he had last year, but it hasn’t mattered, as Bosa has stepped in and immediately become one of the most disruptive pass rushers in the league. They have the ability to attack offensive lines from multiple angles with multiple weapons, taking some of the pressure off their less developed secondary.

There are multiple different ways to build a defense, and if you’re a team looking for a quick turnaround, going that route may be the best approach. Of course, the season is still early, and only time will well what becomes of these teams that have had so much early success. Some like Detroit might regress as it’s revealed their defense isn’t as good as it looks. Others like Buffalo may stay great on defense but be unable to turn that greatness into victories so consistently.

It is something to keep an eye on as the season runs its course, because right now there is only one thing we can say for sure: one way or another, the Patriots are going to find a way to win almost every game they play.

Toss Out the Predictions

We’re four weeks into the NFL season, and everything is more or less going according to plan. The Patriots and Chiefs have coasted to undefeated records. The NFC North is a bloodbath of elite defenses and shaky quarterback play. The Broncos, Redskins, Bengals, Cardinals, and Dolphins all have yet to win a game.

Oh, and also the Eagles, Falcons, Chargers, and Browns are all .500 or worse, the Lions and Bills have only lost the Chiefs and Patriots respectively, and the 49ers haven't lost at all. So maybe not quite everything has gone according to plan.

We enter the season each year with certain expectations, and it never takes long for those expectations to fall to pieces. Of course, four weeks in is still probably too early to say for sure that we should cast aside everything we thought heading into the year. Four weeks into last year the Colts, Cowboys, and Chargers were disappointments, while the Bengals and Dolphins were pleasant surprises. Things can change quickly in this league, and they can just as quickly change back.

So at what point in the year do we start trusting what our eyes are seeing? At what point do we cast aside everything we thought we knew coming into the year, and at what point do we believe what has unfolded on the field?

There are a couple different ways to look at preseason expectations. The simplest is to just look at how the team did a year ago. After all, NFL rosters don’t turn over that much from year to year, and generally good teams stay good and bad teams stay bad. This is obviously a huge oversimplification, but there is some value to it. Year over year, a team’s final record correlates 32% to their record from the season before. (All calculations in this go back to the 2000 season.)

Of course we know a lot of stuff does happen in the offseason, and there is an easy metric available to see what the expectations were before the season, factoring in both a team’s success from the previous year and changes they made during the offseason. I was able to pull the preseason Vegas win over-under values for each team going back to 2000, and I compared them with the records these teams realized at the end of the year. The correlation was 40%, better than just by looking at the previous year’s record but not dramatically so.

So if we can get correlations with the final season record of 30-40% before the year begins, at what point can we do better just by looking at the results from within the season? Knowing now that the 
Bills are 3-1, is that more or less meaningful than knowing they were predicted to win 7 games at the beginning of the year?

The table below shows the correlations achieved by comparing each team’s record at the end of each week with their record at the end of the year (again for a sample from 2000-2018).

Week	Record to this Point
1	34%
2	45%
3	57%
4	62%
5	68%
6	74%
7	77%
8	81%
9	85%
10	89%
11	91%
12	93%
13	94%
14	96%
15	97%
16	99%
17	100%

As you can plainly see, it doesn’t take long for the records within the season to eclipse the predictiveness of the preseason expectations. The outcome of the very first week is more meaningful than the record from all 16 games of the previous year, and by the second week the correlation is already higher than using the preseason win-loss totals.

Of course, there’s a pretty clear flaw in this analysis. The games a team has won through the first three weeks count towards the season end total. The 49ers are 3-0, which means there is no chance of them finishing 2-14. It means that to finish with the 8-8 record that Vegas predicted before the season they would have to play significantly below .500 from this point on.

So that raises the next question: how effective are these varying metrics at predicting the performance of a team for the rest of the season? Does the fact that a team like Buffalo has started off hotter than expected mean that they will carry this for the rest of the year, or is it more meaningful that they were expected to be a below average team coming into the season?

The table below has three columns. The first shows how a team’s winning percentage at the end of each week correlates with its winning percentage over the remainder of the season. The second column shows how the winning percentage of the rest of the year correlates with the record from the previous year. And the third column shows how the remainder-of-year winning percentage correlates with the preseason Vegas win totals.

Week	Record to this Point	Last Season Wins	Preseason Win Total
1	19%	31%	43%
2	24%	30%	43%
3	32%	29%	42%
4	33%	28%	41%
5	36%	27%	40%
6	40%	25%	37%
7	41%	24%	36%
8	43%	23%	35%
9	44%	23%	34%
10	48%	23%	34%
11	47%	21%	32%
12	45%	21%	30%
13	37%	20%	28%
14	34%	19%	26%
15	28%	14%	20%
16	24%	19%	21%

Four weeks into the season, we already know more about the rest of the season by looking at each team’s record than we do looking at their record from 2018. And yet, the Vegas win totals are still more meaningful predicting the rest of the season than each team’s record to this point. We need to wait for two more weeks before we reach the point that the two flip, and what we have seen in the season so far becomes more predictive than what we expected coming into the season.

More than any other sport, football is chaotic and random. A sixteen game season, with a quick aging curve and high injury risk, all combines to make it thoroughly unpredictable. Typically nearly half of each playoff field turns over from one year to the next. And yet, despite all this, there remains a great deal of value in looking at our expectations from before the season, value that lingers even weeks after games have started to be played.