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Don’t Blame Defense for Offensive Regression

| January 13th, 2020

Chicago scored only 17.5 points per game in 2018, the 4th worst mark in the NFL. This was a significant drop from 2018, when they were 9th best in the NFL at 26.3 points per game. I’ve seen some people argue that this is mainly due to Chicago’s defense, which scored 5 fewer touchdowns and forced 17 fewer turnovers in 2019. The logic then was that the offense in 2018 was just as bad, but it was overshadowed by a dominant defense that handed them points on a regular basis.

This argument made me curious, so I dug into the numbers to see if it held up.


Non-Offensive Points

The 2018 Bears had 44 points that were scored by their defense and special teams, while the 2019 version had 16*. If you remove those from the season totals, the 2018 offense scored 277 points (23.6 per game) while the 2019 version scored 264 (16.5 per game).

That means Chicago’s offense scored 7.1 points per game more in 2018 than 2019. The total points scored dropped by 8.8 per game, so clearly the bulk of that was from offensive points, not defense/special teams.

*Quick disclaimer: I tallied touchdowns, field goals and safeties, and applied values of 7, 3, and 2, respectively, for each. These numbers might be a little off because not all touchdowns result in exactly 7 points due to missed extra points or going for 2.


Points Off Turnovers

Of course, points directly scored by the defense is only half of the original argument. The 2018 defense also forced far more turnovers, in theory setting the offense up in better field position for more easy points. In order to see what effect this had, I looked at points scored off turnovers for each team using Pro Football Reference’s Drive Finder. The table below shows the data.

Despite forcing far more turnovers, the Bears didn’t actually get many more points from them. Part of this may because of where the turnovers happened; the average starting field position for these drives was the Chicago 42 in 2018 and the opponent 47 in 2019. Accordingly, the Bears got 2.4 points per drive off turnovers in 2018 and 3.3 in 2019. Because of this, offensive scoring following a defensive turnover changed by only 0.7 points per game from 2018 to 2019.

This means that the offense scored 19.6 points/game on drives that did not start off turnovers in 2018, and only 13.7 points/game on such drives in 2019. That’s a difference of 5.9 points per game, roughly 2/3 of the dip in scoring that we saw in 2019. Thus it’s safe to say that the defense’s dip in turnovers did contribute some to Chicago’s scoring woes, but offensive regression was a larger factor.


Long Drives

Of course, it could still be argued that Chicago’s defense led to easy points for the 2018 offense even when they didn’t force turnovers simply by stopping the opponents more quickly and thus setting the offense up in better field position. To explore this possibility, I compared how the offense performed on drives starting inside their own 30, leaving them with a long field to get points. This data can be seen in the table below.

Here again we see evidence that the 2019 offense was significantly worse than its counterpart regardless of any defensive contributions. Once again we see a decrease of roughly 6 points per game, suggesting that roughly 2/3 of Chicago’s scoring regression came from the offense, completely independent of the defense.


Conclusions

There is definite truth to the argument that Chicago’s defense contributed to the Bears scoring fewer points in 2019 than 2018. They scored 5 fewer touchdowns and handed the offense fewer easy points on short drives off turnovers. The spate of turnovers and defensive touchdowns made the offense look a bit better than it actually was in 2018, which is why they were 9th in points scored despite being somewhere between 15th and 20th in most other offensive metrics (16th in yards/drive, 17th in 3 and outs, 14th in drive success rate, 20th in yards/play, 20th in DVOA).

Even after you remove those defensive contributions to scoring, however, we find that the majority of the regression we saw in 2019 was due to the offense. They were legitimately one of the worst units in the NFL (29th in yards/drive, 29th in 3 and outs, 27th in drive success rate, 31st in yards/play, 25th in DVOA). The offensive regression was significant and 100% not to be blamed on the defense.

The Bears did not have a bad offense in 2018 (though it also wasn’t particularly good). They had a terrible offense in 2019. That’s why they finished 8-8 and didn’t make the playoffs despite having one of the best defenses in the NFL. It’s also why their team regression in 2019 was so startling, as there was nothing to suggest their offense was ready to implode following the 2018 season.

Now that we have that established, I’ll look separately at the rushing and passing attacks to see what changed to cause that regression in the next few weeks.

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