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Aggressiveness Index 2020 | Football Outsiders

6 min read

Football analytics people have been beating the drum for fourth-down aggressiveness for two decades now. The good news from the last three years is that, slowly but surely, we’re winning the argument.

Football Outsiders introduced the concept of Aggressiveness Index way back in Pro Football Prospectus 2006. The goal was to find a way to rank coaches based on their tendencies on fourth downs in a manner that was easy to understand but accounted for the different rates at which the average coach will choose to “go for it” in different situations. Although no NFL coach is as aggressive as the data suggests he should be, we discovered there is quite a wide range of fourth-down tendencies among coaches.

Aggressiveness Index numbers center around 1.0 and generally describe how much more (or less) likely each coach is to go for it on fourth down compared to his peers; for example, a coach with 1.20 AI is roughly 20 percent more likely to go for it than an average coach historically would in equivalent situations. There are other methodologies now for measuring fourth-down aggressiveness, mostly based on win probability analysis: for example, the EdjSports head coach rankings or results spit out by Ben Baldwin’s fourth-down simulator. Each methodology will have small differences in how it ranks the coaches, but Aggressiveness Index differs from the others by measuring coaches not against what they should do but against the actual decisions made by coaches themselves.

That’s based on historical averages, of course; the numbers on aggressiveness have been much higher over the last three years. Take a look at the leaguewide numbers for the last seven seasons. This year, the overall attempts on fourth down were 60% higher than expectation based on historical norms.

NFL Aggressiveness Index, 2014-2020
Year AI Overall
2014 0.98 197 201 3217 6.1%
2015 1.00 211 212 3265 6.5%
2016 1.06 233 220 3193 7.3%
2017 1.03 215 209 3259 6.6%
2018 1.37 272 199 3049 8.9%
2019 1.46 307 210 3086 9.9%
2020 1.60 386 242 2884 13.4%

The Aggressiveness Index excludes obvious catch-up situations: third quarter, trailing by 15 or more points; fourth quarter, trailing by nine or more points; and in the last five minutes of the game, trailing by any amount. It also excludes the last 10 seconds of the first half, and it adjusts for when a play doesn’t actually record as fourth-and-short because of one of those bogus delay of game penalties that moves the punter back five yards. Only regular season is included.

One thing you might notice is that both actual fourth-down attempts and expected fourth-down attempts were up in 2020 despite overall fourth-down opportunities being down. This is what happens when offenses are at their highest level in league history. Offenses were more likely to get a new set of downs before ever reaching fourth down, which meant fewer overall opportunities. But when offenses did reach fourth down, they were more likely to get eight or nine yards before they got there, which meant more expected attempts. Total qualifying fourth-and-1 opportunities went from 302 in 2019 to 366 in 2020. Total qualifying fourth-and-2 opportunities also went up, from 202 to 229.

The league averages for aggressiveness have changed so much over the last three seasons that we may need to re-figure the baselines at some point in the near future. Eventually, we’re going to end up with two different versions of Aggressiveness Index: one that’s centered around 1.0 for the years since 2018 and one that’s centered around 1.0 for history so that we can still compare current coaches to coaches of the past.

Aggressiveness Index comes out with Kliff Kingsbury of Arizona as the most aggressive head coach of 2020, at least when compared to historical averages. Kingsbury’s AI of 3.12 is the second highest ever measured, trailing only John Harbaugh’s 3.95 in 2019. Doug Pederson’s AI of 2.64 is the third highest ever measured. 11 of the 20 historical seasons over 2.0 (minimum 60 opportunities) have taken place since 2018.

Matt LaFleur of Green Bay went for it the most often on qualifying fourth downs, 25% of the time. However, Kingsbury finishes No. 1 in AI because of some very unexpected go for it decisions in fourth-and-mediums. Four times, Kingsbury had Arizona go for it on fourth down with 3 or more to go before the fourth quarter while losing by less than 10 points. He also went for a fourth-and-12 in Week 13 from the Rams 40 while losing by 10, and a fourth-and-4 from the Washington 37 in Week 2 in the third quarter of a game Arizona was winning.

Kingsbury is a surprising No. 1 in AI because he is not judged kindly by WPA-based measures of fourth-down aggressiveness. He went for it a lot, with a league-leading 21 fourth-down gos that qualified for AI. But those were balanced out by some egregious errors, in particular the decision to have Zane Gonzalez try a 49-yard field goal with 1:58 left, losing to Miami by 3 in Week 9. Both the EdjSports model and Ben Baldwin’s model had this as the worst fourth-down mistake of the season, and that counts a lot more in a WPA-based measure than it does in Aggressiveness Index, where every decision has the same weight.

An even more surprising rank in 2020 AI may belong Frank Reich of Indianapolis, who has a reputation for being very aggressive but somehow comes out only 23rd in AI. It turns out that Reich went for it a lot in 2020 because he had a lot of opportunity to go for it. Mike Tomlin, who is not known for being an aggressive head coach, ends up in roughly the same boat as Reich. Reich had 13.2 expected gos based on historical baselines. Tomlin had 12.4, while no other coach was above 11. Reich went for it on 14-of-21 qualifying fourth-and-1s. Tomlin went for 12-of-22 qualifying fourth-and-1s. No other coach was above 17 qualifying fourth-and-1s.

Where Indianapolis didn’t go for it was in the unexpected situations, such as fourth down with more than 2 to go. In the first three quarters, the Colts only did this once for the entire season: Week 13 against Houston, when the Colts went for fourth-and-4 on the Houston 39 in the second quarter and Philip Rivers threw a 39-yard touchdown pass to running back Jonathan Taylor on a wide-open wheel route.

With the dramatic change in frequency of going for it on fourth down over the last couple years, 33 of 35 head coaches come out with an AI above 1.0. The exceptions are Vic Fangio and Brian Flores. Flores is a surprise because he was ranked ninth in his first season, 2019, but he had a lot of fourth-and-1 opportunities last year and didn’t use them wisely, going for it in just 6 out of 17 qualifying opportunities. Fangio is easily last in Aggressiveness Index for 2020, and his AI of 0.54 would have been very low even for one of the seasons before 2018 when the whole league was going for it on fourth down less often. In a couple of days, I’ll be running another article with historical Aggressiveness Index numbers going all the way back to 1983. There have been 192 men since 1983 with at least one year as an NFL head coach, and out of that group Fangio’s AI over two seasons ranks 185th. Denver only went for it four times last year in the first three quarters of a game. One of those was a fourth-and-5 against Kansas City, losing by 18 in the third quarter, so it doesn’t count for AI. The other three times were fourth-and-1, and unfortunately the Broncos didn’t pick up any of them.

Here are the full Aggressiveness Index results for 2020.

Thanks to Jim Armstrong, who came up with the idea of Aggressiveness Index and computes it for every season. Later this week, we’ll return with a look at historical Aggressiveness Index going all the way back to 1983.

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