Author: CJ Miller (cjmiller2023@u.northwestern.edu), Weinberg

A hot topic in today’s NFL is the importance of each individual position. It is undoubtedly necessary to have positional priorities straight for GMs, as they operate under a hard cap. Someone like Bears GM Ryan Pace has made enemies out of his Chicago fanbase by trading away a plethora of draft picks for elite pass rusher Khalil Mack, leaving his team stranded in mediocrity, after missing on QB Mitch Trubisky. Meanwhile, Miami Dolphins GM Chris Grier used his draft capital to build a great secondary, which has led to the Dolphins’ remarkable 2020 turnaround. While many (including the MVP voters) will agree that the quarterback is the most important position, what comes next? And what is the financial makeup of the perfect team? To begin to answer this question, we can analyze the effect of the pass rush and the pass coverage on stopping the most important part of an offense.

Turn on any game of professional football today and you are virtually guaranteed to hear the announcers discuss the “battle of the trenches”, referring to all the action around the line of scrimmage. They emphasize this as a key predictor of which way a game will swing. This sentiment is reaffirmed when the national public sees the frontrunners for Defensive Player of the Year, who are big linemen and edge rushers like Aaron Donald, Myles Garrett, and T.J. Watt. But just how important are these players? The first step of answering a question like this one is how they affect the most important facet of football: the passing game. Defensive linemen will rush the quarterback in an attempt to pressure or sack him, preventing an accurate throw from ever getting to their receiver. This can undoubtedly alter plays, but how crucial is a good pass rush?

To find out how much pass rush matters to the overall pass defense, we can attempt to find the correlation between how good a pass rush is and how successful the defense is at stopping an air attack. To do this, I have constructed two scatterplots.

The metrics I have used to measure pass rush success are pass rush win rate, which measures how often a lineman is able to beat his block in under 2.5 seconds, and sack percentage, which is how often a passing play results in a sack. These two data points combined can give us a great idea of how good a team is in the trenches, because it tells us how often their men shed blocks, and how often they can take the quarterback down after they do so.

For measuring overall pass defense success, I used average opponent QBR, which tells us how efficient their opponent’s passing attack was, and net yards per attempt, which shows the average number of yards allowed by a team when their opponent intends to pass. Both of these depict a pretty holistic view of how well the competing team is passing the football.

We see from the first chart that there is a small, weak correlation between a better pass defense and a winning pass rush. R-squared values of .117 and .053 suggest that a more successful pass rush will help your defense, but not by much.

In the second chart we have an extremely high correlation between sack percentage and net yards per attempt, but since net y/a takes sack yardage into account, it is slightly biased. However, we see that sack percentage has a moderate correlation with a lower QBR allowed to the opposition, which can tell us that more pressure on the quarterback leads to a somewhat worse passing game.

From these two charts we can conclude that pass rush does in fact matter, but not to the extent that announcers or popular media might expect. So it is possible that teams who begin their rebuild in the trenches, like the Jaguars or the Bears, are trying to fit a square peg in a round hole.

But the next logical question is, if not pass rush, then what? To make sure that something is actually affecting the passing game of the offensive team, I created another scatterplot using the same y-axes to measure the success of the air attack, but this time I used a different metric which serves the purpose of indicating the competency of the secondary. Pass success rate allowed is the percentage of pass plays that are productive toward gaining first downs*. The lower this number is, the better the pass coverage is doing to prevent efficient catches.

It is clearly evident in the chart that good pass coverage is essential for a good pass defense. The variables are very strongly correlated, and prove that the secondary is a more valuable piece to preventing a passing attack than a pass rush is.

Even more interesting is the actual makeup of the dots on the graph. On the very right, allowing high numbers of yards and QB ratings as well as possessing an atrocious secondary, we have the Jets, Patriots, Texans, and Giants, teams that have not been particularly successful this year. On the other end, we have the Steelers, Chiefs, Dolphins, and Colts, who invested in talented defensive backs and have been reaping the benefits all season long.

Since we see that pass rush does not matter as much as we think it does, to conclude my analysis I decided to check if teams spending more money on defensive linemen have less success. I used win percentage as an indicator of a team’s capabilities, and the share of adjusted 2020 salary cap spent on the defensive line as the independent variable.

When looking at the structure of this chart, one can see that there is not much correlation at all between the amount spent on the defensive line and winning probability. While the trenches don’t have a considerable impact on passing, they do have the primary impact on running, and this is enough to make signing a good lineman worth every penny.

Contrary to popular opinion and old school mentality, my analysis has suggested that pass rush is only slightly important in stopping the pass, and pass coverage is a much better indicator of air attack prevention. However, these same lineman and edge rushers that may not be worth the money during a passing down have a huge impact in the run game. Therefore, spending big on a star defensive lineman may not hurt a team’s chances to win.

*”Success” is defined as a passing play on first down which gains 40%, second down which gains 60%, and third and fourth down which gain 100% of the necessary yardage for a first down.

Sources:

“2020 NFL Pass-Rushing, Run-Stopping, Blocking Leaderboard: Win Rate Rankings.” ESPN, ESPN Internet Ventures, 24 Nov. 2020, www.espn.com/nfl/story/_/id/29939464/2020-nfl-pass-rushing-run-stopping-blocking-leaderboard-win-rate-rankings. 

“NFL Team Sack Percentage.” NFL Football Stats – NFL Team Sack Percentage on TeamRankings.com, www.teamrankings.com/nfl/stat/sack-pct. 

“NFL Team Opponent Average Team Passer Rating.” NFL Football Stats – NFL Team Opponent Average Team Passer Rating on TeamRankings.com, www.teamrankings.com/nfl/stat/opponent-average-team-passer-rating. 

“2020 NFL Opposition & Defensive Statistics.” Pro Football Reference, www.pro-football-reference.com/years/2020/opp.htm. 

“NFL Defensive Line Spending – Cap.” Spotrac.com, www.spotrac.com/nfl/positional/2019/defensive-line/.