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Adjusting team sack rates in the NFL for SOS

I’ve got sacks on the mind today. A few hours ago, I looked at the single-season sack records by various players in both the 3-4 and 4-3. Today I also want to look at things from the team perspective. In 2012, which team is the best at getting to the quarterback? You might think that’s as simple as looking at a list of defensive statistics and sorting by sacks, but I’d like to take a more nuanced approach.

There are two factors that heavily impact a team’s sack rate: the opponents they face and the number of passing plays they defend. Using sack rate instead of sacks helps to solve the latter issue, but we still need an opponent adjustment even if we use sack rate. It’s a bit tricky doing it correctly, because you need to iterate the results just like you do with the SRS. What I mean by that is the sack rate of the Denver defense will need to be adjusted for the sack rate of the Kansas City, Oakland, and San Diego offenses (among other teams), and those will need to be adjusted for the Denver defense (and all the other defenses those teams faced).

Once you properly iterate the results, what are the results? The table below shows the top defenses this year at getting to the quarterback, although note that these numbers exclude the games from week 15. The table shows how many pass attempts each team has faced, the number of actual sacks they have recorded and their actual sack rate; the next column shows the SOS adjustment to the sack rate, with a positive number indicating a difficult strength of schedule (i.e., they’ve faced teams that are difficult to sack). The last two columns show the SOS-adjusted sack rates and total sacks.

RkTeamATTACT SKSK RTSOSADJ SK RTADJ SK
1DEN506397.7%0.9%8.6%43.3
2CIN542437.9%0.5%8.4%45.7
3ARI426348%-0.5%7.4%31.7
4DAL429296.8%0.3%7.1%30.4
5CAR483306.2%0.9%7.1%34.2
6KAN389246.2%0.9%7.1%27.5
7STL489398%-1.2%6.8%33.1
8PHI467286%0.8%6.8%31.6
9HOU533376.9%-0.2%6.7%35.9
10MIA532377%-0.2%6.7%35.7
11GNB524356.7%-0.1%6.6%34.6
12BAL506305.9%0.6%6.6%33.3
13CLE531346.4%0.2%6.6%34.8
14NYG464316.7%-0.1%6.5%30.4
15SFO478326.7%-0.4%6.3%30.3
16PIT434266%0.3%6.3%27.4
17ATL463286%0.3%6.3%29.2
18BUF484316.4%-0.2%6.2%30.2
19CHI498336.6%-0.5%6.1%30.6
20SDG508254.9%1%5.9%30
21TEN481275.6%0.3%5.9%28.3
22SEA475326.7%-1.1%5.7%27
23NWE511285.5%-0.1%5.3%27.3
24NOR496255%0.2%5.3%26.1
25MIN515316%-0.8%5.2%27
26DET490306.1%-1.1%5%24.4
27WAS539234.3%0.7%4.9%26.6
28TAM541244.4%0.5%4.9%26.5
29NYJ446224.9%-0.1%4.8%21.5
30IND470245.1%-0.4%4.7%22.3
31OAK465173.7%0.7%4.3%20.1
32JAX464143%0%3%14

Miller had two sacks against Brady.

Miller had two sacks against Brady.

Let’s go through a couple of examples.

The Rams actually lead the league in sack rate, although I doubt many people would have guessed that so it’s hard to call them overrated. We’ll get to the offensive sack rates in a minute, but the key here is that the 49ers and Cardinals rank last in offensive sack rate (after adjusting for SOS). So four of the Rams 13 games came against the two easiest teams to sack, and the 29th and 30th easiest teams to sack — Green Bay and Chicago — were also on the St. Louis schedule. St. Louis has 39 sacks this year, but 24 of them came in those six games. The Rams produced a 9.6% sack rate against those teams and a 6.3% rate in their other games. St. Louis is far from a fraud, but they’re more accurately rated as the 7th best pass rushing team, not the first.

In the AFC West, the Chargers and Broncos are underrated because of their difficult schedules. That might surprise you, but consider that the AFC West played the NFC South, which featured New Orleans, Tampa Bay, and Atlanta, three teams that rank in the top quarter of the league in offensive adjusted sack rate. Oakland and Denver also rank in the top eight, giving San Diego’s defense two more difficult games, while Denver had two games against the Raiders along with a game against the always-difficult-to-sack Patriots. Cleveland and Pittsburgh also rank as harder than average to sack, and more importantly, there were few patsies on the schedules of either Denver of Oakland. Denver already ranked 4th in sack rate, but once you adjust for SOS, they move ahead of the two NFC West teams and Cincinnati and rank first in the league.

What about the offenses?

RkTeamATTACT SKSK RTSOSADJ SK RTADJ SK
1NYG479153.1%0.1%3%14.4
2NWE519203.9%0.1%3.7%19.5
3DEN503193.8%-0.2%4%19.9
4NOR559234.1%0.1%4%22.6
5HOU471173.6%-0.5%4.1%19.2
6TAM442194.3%0.1%4.2%18.4
7OAK561264.6%0.3%4.4%24.4
8ATL535244.5%0.1%4.4%23.7
9DET618264.2%-0.3%4.5%27.6
10CLE487244.9%0.4%4.6%22.2
11PIT510275.3%0.2%5.1%25.9
12DAL567315.5%0.2%5.3%29.9
13TEN499265.2%-0.5%5.7%28.6
14BUF432255.8%-0.3%6.1%26.2
15IND569325.6%-0.6%6.2%35.5
16BAL482316.4%0.2%6.3%30.3
17MIA442276.1%-0.2%6.3%27.8
18SEA365246.6%0.2%6.4%23.4
19STL464316.7%-0.1%6.8%31.5
20KAN437317.1%0.2%6.9%30.1
21SDG503377.4%0.4%6.9%34.9
22WAS392297.4%0.3%7.1%27.7
23PHI575427.3%0.2%7.1%41
24MIN429286.5%-0.7%7.3%31.2
25CAR425317.3%0%7.3%31.1
26NYJ435317.1%-0.2%7.3%32
27CIN512387.4%0%7.4%37.8
28JAX500408%-0.3%8.3%41.5
29CHI444378.3%-0.4%8.7%38.7
30GNB484428.7%-0.5%9.2%44.3
31ARI555519.2%-0.1%9.2%51.3
32SFO384389.9%-0.1%10%38.3

The Giants lead the NFL in sack rate and adjusted sack rate. The Broncos are the king of the statistic on both sides of the ball, ranking 1st on defense and third on offense. There’s a reason Denver is likely going to enter the playoffs on an 11-game winning streak. Having Von Miller and Peyton Manning helps quite a bit, although check out this article on how Elvis Dumervil is playing a key role for the Broncos this year. In general, SOS matters a bit more when looking at the best pass rushing teams than at the best pass blocking teams (or teams with the best quarterback). This could be an indication that in a given game, the sack rate for each team is more a function of the offense than the defense. I’ve never really looked into that, and that’s a worthy subject for another post. But the key here is that the adjustments simply aren’t that significant.

Philip Rivers and the Chargers rank 26th in unadjusted sack rate, but after adjusting for strength of schedule — two games against the Broncos, a game against Cincinnati, and six more games against better than average pass-rushing teams — they move up to 21st. Von Miller alone had four sacks in two games against San Diego.

What teams move down? The Minnesota Vikings have had a really cushy slate of opposing defenses, which makes Christian Ponder’s struggles even more perplexing. The Vikings played the Jaguars (#32 in sack rate), Colts (30), Buccaneers (28), Redskins (27) and faced the Lions (26) twice, and were sacked 17 times in those six games. In their only game against a top-pass rushing defense, the Cardinals had 3 sacks on just 20 Vikings dropbacks.

I’m going to end with a bunch of disclaimers that are important to keep in mind here. The sack is one of the most misleading statistics in the NFL, especially when looking at individual players. A pass rusher can make a difference on plays by rushing the quarterback and forcing an errant throw (or a sack by a teammate), or by hitting the quarterback on a play. A pass rusher can force a sack by tying up blockers and allowing his teammates to get to the quarterback to record a sack or force an errant throw. When the difference between a great pass rusher is roughly one extra sack per 50 throws, well, “sacks” as a metric simply don’t give us a lot of information.

Also, I suspect there’s a correlation between Game Scripts and sacks per attempt. That’s a complicated issue to resolve, but teams that play with big leads are likely going to get more sacks per opponent pass attempt than teams that are trailing. Perhaps even more importantly, not all passing plays are the same. Football Outsiders calculates an adjusted sack rate which not only adjusts for SOS but for the type of pass play (e.g., it is easier to record a sack on third and long than on second and short) that I highly recommend.

{ 2 comments }
  • Richie December 17, 2012, 3:14 pm

    Chase, I got caught on a sentence in the third paragraph:

    and their actual second rate

    Did that mean actual sack rate?

    Reply
    • Chase Stuart December 17, 2012, 3:21 pm

      Yep – fixed.

      Reply

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