Johnathan Wood | July 15th, 2019

Chicago’s defense was really, really good in 2018. They led the NFL in points allowed, turnovers forced, touchdowns scored, and passer rating against, and finished 3rd in both yards and sacks. They finished as the runaway best defense in Football Outsiders’ DVOA, which is intended to be an all-encompassing metric, and even finished as the 8th best defense ever in DVOA’s database, which runs back to 1986.

Now as we head into 2019, fans are rightly wondering if Chicago’s defense can repeat that performance. While I won’t pretend to be able to predict the future, I can look at the past to see what it might have to tell us. So I looked at top defenses in recent NFL history and measured, through a variety of metrics, where the 2018 Bears excelled. Then I looked to see how they followed that up in the next season. Full data collected can be viewed here for transparency’s sake.

DVOA

The DVOA system is set up such that an average defense gets a score of 0, with negative numbers indicating you are better than average (the farther from 0 the better). The Bears finished with a final score of -26.0, so I looked at other teams in the last decade (2008-17) who finished at -20 or better. This was quite a small list, as it featured only 10 teams. Here’s how they fared in the season following that dominant performance:

Average DVOA : -25.1%

: -25.1% Average following DVOA : -8.8% (8th in NFL)

: -8.8% (8th in NFL) Change : 16.4%

: 16.4% # teams with better DVOA following year : 0

: 0 # teams top 5 in DVOA following year : 5

: 5 # teams top 10 in DVOA following year : 8

: 8 # teams below average in DVOA following year: 1

First, notice that none of these defenses were as good the following year. This isn’t surprising; there were only 10 teams in 10 years who achieved this caliber of DVOA. The odds of doing that twice in a row are very low.

But that doesn’t mean the defenses were bad. Half of them were among the top 5 in the NFL the following year, and 80% of them were top 10. Only one ended up being below average, and that one should be very familiar to Bears fans, as it’s the 2013 squad that completely fell apart after Lovie Smith left and Brian Urlacher retired. That defense lost 3 starters (Urlacher, Nick Roach, Israel Idonije) and had a host of other players on the tail ends of their career who fell apart following 2012. The current Bears are not remotely in that same situation.

If you want to get even more narrowed down, only 5 of these 10 teams had a DVOA of at least -25, which the Bears did in 2018. Every single one of them finished as a top 10 defense the next year, with an average of being around the 5th best defense in the NFL. It turns out that historically good defenses remain good the next year. Shocking, right?

Points Allowed

Now let’s look at teams who finished similar to the Bears in points allowed. Chicago gave up 17.7 points/game (PPG) last year, so I looked at all teams who gave up less than 18 PPG from 2008-17. This is a bit more difficult of a stat to compare across time since scoring has generally gone up over the last decade, but it’s at least some way to narrow it down, and the sample size here was 33 teams.

Average PPG : 16.2

: 16.2 Average following PPG : 20.1 (10th in NFL)

: 20.1 (10th in NFL) Change : 3.9

: 3.9 # teams with better PPG following year : 4

: 4 # teams top 5 in PPG following year : 13

: 13 # teams top 10 in PPG following year : 22

: 22 # teams below average in PPG following year: 7

Here we see similar trends to DVOA. Most – but not all – of the teams fared worse the next year, but they were still generally pretty good. The average defense finished around 10th in points allowed the following year, but that was a lopsided distribution, as 2/3 of the teams finished within the top 10, and over 1/3 of them in the top 5. There were a few bad apples (including the 2013 Bears) who were just really bad the next year that dragged the overall average down.

Turnovers

Finally, let’s look at teams who forced a similar number of turnovers as the 2018 Bears, who had 36. This was actually the first time since 2015 that anybody in the NFL forced 35 or more, but 16 teams overall reached that mark between 2008 and 2014.

Average turnovers : 38.2

: 38.2 Average following turnovers : 26.9

: 26.9 Change : 11.3

: 11.3 # teams with more turnovers following year : 0

: 0 # teams top 5 in turnovers following year : 4

: 4 # teams top 10 in turnovers following year : 8

: 8 # teams below average in turnovers following year: 5

Here we see a bit more of a drop-off. Fewer teams remained at or near the top of the league – though 1/4 were still top 5 and 1/2 top 10 – and the overall average rank the next year was 13th, which isn’t that far from the NFL average (16th-17th for a 32 team league). It makes sense that turnover numbers would fluctuate more than other statistics, as we’re talking smaller sample sizes with fewer turnover plays than points scored or total plays (which all factor into DVOA).

Conclusions

No matter how you look at it, the odds of Chicago’s 2019 defense being as good as their 2018 version are very slim. That’s what happens when you have a historically good year.

But that doesn’t mean their defense is going to be bad. The overwhelming majority of defenses in these samples remained very good the year after their dominant performance, especially when using more all-encompassing metrics like DVOA and points scored.

And here’s the other thing to keep in mind: Chicago’s defense was so much better than other groups in the league in 2018 that they can afford to regress some and still remain the best defense in the NFL. The difference between their DVOA and the 2nd best defense was greater than the difference between 2nd and 10th. The same is true for turnovers, where Chicago had 36 and then the next 11 teams were grouped between 26 and 31.

All of this is a long way to say that yes, the Bears’ 2019 defense is almost certainly not going to be as good as they were in 2018, but history suggests they are very likely still going to be one of the best defenses in the NFL. Which is good news when you consider that history also suggests Chicago’s offense is poised to be significantly better in 2019 than it was in 2018. I predicted after the Mack trade last fall that the Bears would have a top 5 unit on both sides of the ball in 2019, and I still believe that’s possible. My work this offseason has shown that both units are at least likely going to be top 10 if they follow historical trends.