No Pass Rush, No Problem?

The Bears are thin on the defensive edge. Whatever happens with Leonard Floyd’s health this year, the rest of the outside linebacking corps is still looking barren. Akiem Hicks is disruptive on the defensive line, but he’s not that disruptive. This does not, however, suggest that all is lost. Teams can still win the defensive battle without sacks, so long as they get turnovers. And this is good news for the Bears, because despite popular opinion to the contrary, a strong pass rush is not necessary for (or even connected to) a team’s ability to generate interceptions.

Before continuing, however, I have been advised that I must include the following announcement:

WARNING!

This article will look at multiple results across many years, as well as provide some original analysis of raw data in order to draw conclusions. If you would rather trust your gut, know what you know, or disregard evidence in favor of truths, this will likely be a very troubling piece for you. Please feel free to skip directly to the comments section and make a denigrating comment about analytics, likely involving an anecdote of a time analytics might or might not have been used that resulted in a negative outcome. For bonus points, mention how running sets up the pass.

Okay, are we back? Great.

So, I keep hearing commentators and fans espouse the belief that pass pressure is necessary to generate turnovers. “You can’t give the quarterback all day to throw and still expect interceptions,” or some other bit of wisdom. This seemed strange to me, because I remember a long run of seasons post-Super Bowl XLI when the Bears had really good interception rates and perfectly mediocre sack rates. I was also aware of one great study on the subject, but before I reread it, I decided to investigate just a little bit on my own.

2017: Sacks did not yield interceptions

Not wanting to reinvent the wheel but curious about what last year would show me, I looked at the numbers (not rates) to see if I could tease out any sort of difference in the numbers. The average sack total for 2017 was 37.3, with a standard deviation of +/- 8. Basically, for those who don’t like to play this math game, any number between 29 and 45 was part of the ‘middling’ performance group*.

The best teams in terms of sack totals were the Rams (48), Panthers (50), Jaguars (55), and Steelers (56). The first two were a full standard deviation above the mean, but those last two are notable because they are actually more than two standard deviations outside of the average. That’s impressive, and tough to gloss over. Meanwhile, the Jets (28), Bills (27), Giants (27), Colts (25), and Buccaneers (22) were all quite bad, although none as as far below the average as the Jaguars and Steelers were above it.

Okay, so how did these teams do when it game to generating interceptions? The average NFL team picked up 13.4 interceptions last year (standard deviation was 4.3). There was not a lot of consistency when it came to having a lot of sacks and a lot of interceptions. Pittsburgh was functionally average with 16, but Jacksonville was solid with 21. The Panthers were at 10, below average but not notably so, while the Rams managed 18 picks.

Two of the four top teams in terms of sacks were also top teams in terms of interceptions, but the other two leaders (including the absolute leader in sacks) were nothing special. The actual leader in interceptions (Baltimore) had 22 picks but only 41 sacks, and Detroit was 5th in interceptions but had an almost perfectly middle-of-the road sack total of 35.

The flip side is equally important to consider, though, because it’s a lack of pass rush that Bears fans are worried about. The worst teams in terms of sack rate included Buffalo, which came in with 18 interceptions (more than sack-leader Pittsburgh, and a full standard deviation above average). Remember the Giants, Colts, and Bucs with their paltry sack totals? They suffered, right? Nope. They all posted perfectly average interception totals of 13 each.

This should not be surprising to Chicago fans. The Bears had 42 sacks in 2017, more than all but six teams in the NFL. They were also one of the worst in the league in interceptions. Clearly, we need more than one year’s worth of date to look at this myth.

One of the best studies nobody reads

To look at the impact of the pass rush on turnovers, there’s one fantastic source that explains it better than most. That’s Ed Feng’s article at The Power Rank, titled “How to Predict Interceptions in the NFL, Backed By Surprising Science.” It’s an impressive little bit of research, it explains a lot about turnovers, and it seems to have been largely ignored by a majority of football analysis. [LINK]

Dr. Feng looked at eleven seasons of NFL defenses starting in 2003 and going through 2013. He used sack rate to see whether or not a team’s ability to sack the quarterback and apply pressure had an impact on turnover rate: “While I expected defenses with a better sack rate to have a higher interception rate, there’s no correlation between these two quantities for these 352 defenses.” That’s worth repeating: looking at 11 years of data, he found that the sack rate a defense managed was essentially negligible in its impact on their turnover rate.

Dr. Feng did not give up so easily, though, and so he cross-checked his data by using QB hit rates over five years (basically, as long as he had access to the data for, from 2009-2013), and he found that quarterback hits only accounted for four percent of the variance in interception rates. That’s ... not a lot. The nice thing about using quarterback Hits is that it’s a much larger number. Sacks are ultimately rare events, but quarterback hits happen much more often. However, even then, pass pressure was not a reasonable predictor of interception rate at all.

Just as my own weak analysis predicted, the study by The Power Rank showed there was not a real connection between these two defensive accomplishments. What’s interesting is that while I used a small data set and went at it from a raw total perspective, Feng used rate to refine his data and looked at a much larger data set. Whether someone was looking at the forest or the trees, though, the results were not in favor of a link between interceptions and pass pressure.

Any Defensive Stop is a Good One

It’s worth pointing out that both types of defense production present paths to winning. The best five teams in terms of interceptions last year assembled 52 wins with a 65 percent winning percentage. The worst record among them was 9-7. The worst five teams in terms of interceptions last year assembled 27 wins with a 34 percent winning percentage. The Atlanta Falcons managed a winning record, but the other teams were the Browns, Bears, Raiders, and Dolphins. A lot of interceptions do seem to correlate with a winning record, while lacking interceptions does seem to make it harder, but not impossible, to win.

Likewise, the best four teams in terms of sacks last year (because that’s the cutoff for the standard deviation, but also because a lot of ties start happening) assembled 45 wins and a 70 percent winning percentage. Meanwhile, the worst four teams in terms of sacks last year managed 21 wins and a 33 percent winning percentage. The Buffalo Bills managed a winning record, but as has been noted they had an exceptional turnover rate. Just as with interceptions, sacking the quarterback paid dividends. And failing to sack the quarterback had consequences for most teams, but not for every team.

Do the Bears need help at pass rush? Yes, they do.

Does a weaker pass rush mean that they will fail to generate turnovers? There is a lot of evidence that says—regardless of fan perception—that pass pressure is not required for interceptions and that the two present independent. That’s opposed to intrinsically linked, paths to success.

For Bears fans, this was bad news last year. The Bears got the sacks, but not the interceptions. This year, however, it could be the other way around. Even without consistent pass pressure, the Bears could win the turnover battle. That ultimately could mean that the defensive backs are what will turn this team around. With Vic Fangio aligning the Bears in nickel defense almost two-thirds of the time last season (9th highest in the NFL, per Football Outsiders), it’s possible that Chicago’s defensive coordinator has a plan for exactly how this team can win.

Unless otherwise noted, my raw data came from Pro Football Reference.

*To fellow number nerds: Yes, this is a sloppy explanation. But I’m trying to convince people math isn’t scary. So work with me!