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Advanced Defensive Stats: Pass Rush

| July 14th, 2020

I’m continuing to look at Chicago’s defense using advanced defensive statistics from Pro Football Reference (PFR). I already looked at missed tackles and coverage, and today I want to look at pass rush.


Expected Sacks

In general, sacks are fairly variable from year-to-year due to their small sample size. Accordingly, they are not a very good way to evaluate a pass rusher, just like rushing or receiving touchdowns (which also have a small sample size) are not the main way we evaluate skill position players.

This is where advanced statistics can give us a more helpful overall picture of a pass rusher’s performance. The PFR database tracks total QB pressures, which gives you a larger sample size and thus should be more reflective of the player’s performance.

I was curious about the relationship between total pressures and sacks, so I took the following steps to investigate:

  • I examined all rushers between 2018-19 (the only 2 years this database has) who had at least 15 pressures in a year; I chose this threshold to look only at full-time pass rushers. This gave me a data set of 215 seasons, or roughly 3.4 rushers per team per year.
  • I found that the typical ratio was 3.8 pressures per sack, though this had a very high standard deviation (4.0), highlighting how much it varies from person to person.
  • When I looked only at 30+ pressures in a season (63 samples, so roughly 1 player per team per season), the average stayed virtually identical at 3.7 pressures per sack, but the standard deviation dropped to 1.2. This suggested to me that the typical number of around 3.8 pressures/sack is legitimate, and the high standard deviation with the 15 pressure cutoff was largely due to small sample sizes; you get lots of fluctuation in pressure/sack ratio when the pressure number is small.

Using that 3.8 pressures/sack as the norm, then, you can come up with how many expected sacks a player has for a season. If a player has 38 pressures, they are expected to have 10 sacks (38/3.8). You can then easily get a sack differential; a player with 10 expected sacks who actually posted 7 would have a differential of -3, indicating they were 3 sacks below what they should have normally had.


2019 Bears

I included all DL and OLB who registered pressures in 2019, as well as Robert Quinn and Barkevious Mingo. Players with a sack differential of +1 or better are highlighted in green, while those with a sack differential of -1 or worse are highlighted in red. I also included 2018 data to give you an idea of whether 2019 results were consistent with the year before.

A few thoughts:

  • There’s a whole lot of red in that chart. Overall, the full-time pass rushers in 2019 were about ten sacks short of expectation. This was a stark contrast to 2018, when those same players (at least those of them who were on the Bears) were about 2 sacks short of expectation.
  • Those 2 sacks short in 2018 came from Leonard Floyd, who also came up the farthest below expectation in 2019. Whether this is an indictment of Floyd or just random bad luck will be explored below. Either way, Robert Quinn consistently generates more pressure than Floyd, making him a clear upgrade in the pass rush department.
  • Speaking of Quinn, he outperformed his expected sacks last year, so don’t be fooled by that large 11.5 number. He’s probably an 8-9 sack per year guy based on his performance the last 2 years.
  • The one Bear who massively outperformed expectations last year was Nick Williams, who had 6 sacks despite generating fairly minimal pressure that expected him to produce roughly 3 sacks. That earned him a decent contract in Detroit (and a 2021 compensatory pick for the Bears), and I don’t think Chicago will really miss him.
  • If you want to know how good Khalil Mack is, there are only 19 seasons across the last 2 years with 45 or more pressures, and Mack has 2 of them. He’s one of only 3 players in the NFL (along with Aaron Donald and Cameron Jordan) to generate at least 40 pressures in both years.
  • On average, teams have 1 player get 30 or more pressures per year. Khalil Mack and Robert Quinn have both performed at that level over the last 2 years, while Akiem Hicks had 29 in 2018 and was on pace for right around that while playing 5 games through injury last year. If that trio can stay healthy, Chicago’s pass rush will be fun to watch.
  • Depth, on the other hand, is a concern. There is nobody of note behind Mack and Quinn; Isaiah Irving and Barkevious Mingo don’t exactly offer much as pass rushers. On the defensive line, Roy Robertson-Harris provides some nice pass rush, but his snaps jumped by almost 200 from 2018 to 2019 without any increase in pressures. Bilal Nichols saw his snaps jump by over 100, but he generated less pressure than in 2018. If any of Hick, Mack, and Quinn get seriously hurt, the Bears’ pass rush could start to slip pretty quickly.

Repeatability?

Finally, I want to look at how repeatable sack performance vs. expectation is year over year. Just from looking at the table above, my guess was not very, but it helps to look at the data and confirm.

To try and minimize noise from small sample sizes, I looked at all players with at least 20 pressures in both 2018 and 2019. This gave a data set of 45. An examination of sack differential in 2018 vs. 2019 showed very little relationship, as you can see in the graph below (reminder: r2 ranges from 0 to 1, with 0 being no relationship and 1 being a perfect relationship).

Of the 45 players, only 7 had a sack differential of +1 or greater both years, and only 4 (including Floyd) had a sack differential of -1 or worse in both seasons. Only 2 seasons of data is too small of a sample size to say for sure, but it looks as though there is very little repeatability from year to year for players who over or under perform their expected sack totals.

That’s good news for the Bears, who got far fewer sacks in 2019 than their pressure numbers indicate they should have. They should bounce back nicely in 2020, and that’s before factoring in the return to health of Akiem Hicks or addition of Robert Quinn.

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