• Bruin Sports Analytics

Analyzing Fourth Down Decisions in the 2021 NFL Season

By: Victor Li

Source: NFL.com

Arguably the most unique down in American football is the fourth down. While teams get to attempt another play (as long as they maintain control of the football) after the first three downs, the fourth one is the last. If a team maintains control of the football after a fourth down but does not get another first down or score, they turn the ball over.

The Main Fourth Down Decisions

Because of the unique situation of fourth downs, there are multiple distinct options teams can have. In the first three downs, teams virtually always run plays to attempt to gain yardage (aside from scenarios where there isn’t much time left on the clock). However, since the opposing team takes over at the spot of the play after a failed fourth down, teams are heavily incentivized to either punt or attempt a field goal.

The goal of a punt is to simply kick the ball so far away that when your opponent receives it, they are much further away from the end zone they wish to reach. Because of this, punting is often the default option for teams on fourth down.

The other common option is a field goal, in which teams try to score 3 points. However, a missed field goal doesn’t advance the ball, leaving the opposing team with very good field position. Thus, field goals are generally only taken when teams are relatively confident that their kicker can make the field goal. This often depends on distance, so teams often have a “field goal range,” in which they would kick field goals within the range and punt outside of it. Currently, this range is usually around 52 yards — which seems like a lot — but, this range also has to take into account the entire end zone (since the uprights are at the back of it) and an extra 7 or so yards from the line of scrimmage to where the kick will actually take place.

That leaves the third option: just go for another first down, and deal with the consequences if you don’t get it. It’s basically like going all in during Texas hold’em. These fourth down conversion attempts can be some of the wildest and craziest trick plays you’ll see, or they could just be a quick QB sneak from 6’5” Josh Allen. I think these plays generally reflect on the coaching staff. Third down plays are often standard plays you’d expect to see many times over the course of the game, but the do-or-die nature of fourth downs means that coaches are heavily incentivized to use their single most effective play to gain the required yardage in one go, even if that play cannot be repeated (like a trick play) or is risky in general. In addition, teams almost unanimously opt to aim for the first down marker and nothing more (meaning no deep throws), meaning that the defense knows exactly where the offense wants to be; it’s up to the play caller to figure out how to get through anyway.

The bigger influence of the coaching staff, however, first comes from even deciding to go for it in the first place. Barring desperation, teams must have a good reason to depart from the standard punt-kick system. The distance to the next fourth down is the most obvious one: a 4th and 1 is theoretically much easier to convert than a 4th and 9. There’s also the teams themselves: some teams are simply better at converting tough downs than others, while some teams are better at defending fourth downs as well. Finally, there's the game situation. Sometimes, a 3 point field goal will do you no good if you’re down by 4. Other times, coaches will hedge their bets on the potential of a fourth down to either turn the tides of a game or secure their control over it.

We’ll attempt to look at some factors that could cause teams to go for it on fourth down, as well as whether or not those teams made the right decisions.

Which teams “go for it” the most often?

Through week 14 of the 2021 NFL season, we can get the following graph for how often teams attempt fourth down conversions or punt. We’ll exclude field goal attempts for now, as they occupy a sort of “middle ground” that isn’t just giving up but also isn’t going all in.

Immediately, we can see a couple standouts:

  1. The disappointing Seahawks don’t choose to attempt fourth down conversions often — for good reason, as they have one of the lowest conversion rates as well.

  2. Most of the hyper-aggressive fourth down teams (Detroit, Washington, and Cleveland) have below average conversion rates. This is expected, as these teams probably attempted more fourth down conversions in difficult situations.

In general, there seems to be very little correlation between how often teams go for it on fourth down and how well they can convert it when they try. Some of this may be due to sample size, but I think it also suggests that coaches generally DON’T consider how good their team is on fourth down when making their decisions. Some teams, like the Vikings and Patriots, could probably benefit from more aggressive fourth down play calling. On the other hand, the afore-mentioned super-aggressive teams could probably be more conservative on fourth down.

When we include field goals as well, the teams all obviously attempt conversions less (since field goals are now included options). However, relative to the rest of the league, most teams remain in roughly the same spot. The key differences seem to be that the outliers become even bigger outliers.

We can see the Chargers go from arguably the best fourth down team in the league to clearly the best (at least statistically), with no other teams in its vicinity in terms of both conversion rate and tendency. The hyper-aggressive teams in general remain even more aggressive by comparison, as well. In both cases, this suggests that these teams amp up their aggression within field goal range, settling for 3 points much less than the rest of the league.

On the other hand, the Patriots seem even more passive: in true New England fashion, they are one of the most content teams in the league with kicking field goals (maybe because Belicheck doesn’t trust his quarterback).

Fourth Downs and Winning Games

We might now think to see how well fourth down tendencies correlate with winning:

No matter whether we include field goals or not, there is little to no correlation between how aggressive a team is on fourth down and how often they win. However, if we instead consider the teams’ fourth down conversion rates:

Finally, there is some correlation. In general, the teams that win more are better on 4th down (and vice versa). As a side note, the Arizona Cardinals were 100% on their 4th down conversion attempts during their 7-0 start. As for why we see this correlation, there are a couple possible explanations:

First off, strong offenses would naturally be better at converting fourth downs. We can see that many of the teams in the top right quadrant (Tennessee, Arizona, Green Bay, Baltimore, Kansas City) all have strong offenses — their strong offenses mean they win lots of games, as well. On the other hand, teams like the Seahawks, Texans, and Bears have poor offenses and thus have poor conversion rates as well.

Another explanation is that winning teams with good conversion rates simply make better decisions on fourth down. We’ve discussed the potential drawbacks of failing on fourth downs, so the teams who correctly assessed their chances on fourth down and made good decisions should thus have more success in winning.

In either case, we need more context in order to properly analyze fourth down decisions.

Field Position and Yards Until First Down

Clearly, field position and yards until first down are highly important with fourth downs. Successful fourth down conversions become more valuable the closer a team gets to the opposing end zone, and it’s easier to convert when the distance until first down is smaller. We can see the relationship between field goals and punts as well.

Some takeaways:

  1. We can see a pretty clear separation between green and red at around the 33 yard line (field position). This represents a pretty consistent field goal range across the league: there’s almost no red to the left of this line, whereas, there's almost no green to the right.

  2. There’s also lots of blue dots just scattered all around the graph — most of these are probably either end-of-game desperation plays or really bold play calling.

  3. As expected, there are more blue dots (conversion attempts) with short distances until first down. These seem to be most prominent:

  4. within the 10 yard line (when teams smell blood and are confident that another first down will lead to a touchdown) and

  5. near/past the field goal range boundary but not beyond 60 or so yards (where teams won’t feel too good about either a field goal attempt OR a punt — more on this later).

  6. In between these areas (10 - 30 yards), there seem to be mixed calls between going for it and kicking the field goal - this is probably where many meaningful decisions come in.

  7. Interestingly, there also seems to be a huge cluster of blue dots (and mixed-color dots) around the field goal range (33 yards) with around 1-5 yards until first down. As previously mentioned, 4th and 1 attempts seem to be very common close to the end zone. However, this area seems to be the true hotspot for conversion attempts, since we see not only 4th and 1s but also 4th and 5s, 6s, and beyond. I think the reason for this is that the opportunity cost of failing the conversion is at its lowest here. The 3 points you could get from a field goal aren’t guaranteed, as it’s at the edge of the field goal range. However, you also aren’t giving your opponents really good field position either, making this the least risky spot to go for it. In addition, a successful conversion means you are almost guaranteed to get at least a field goal, meaning the reward is pretty solid. As such, this is another area where teams/coaches can show their aggressiveness on fourth down.

  8. The dots at 4th and 10 (10 yards until first down) within 25 yards of the end zone are almost all pure green, suggesting that teams generally just opt to take the essentially guaranteed three points in this scenario. A 4th in 10 will generally only happen when your team picks up a first down and then gains 0 yards in their next three attempts; not trusting your offense here seems to make sense. Many of these situations probably also occur when the offense got well within field goal range but then rushed to try to get a touchdown immediately, resulting in a quick 4th and 10.

  9. Finally, if you get further from the end zone along this 4th and 10 line, the decisions become much more varied, with lots of cyan magenta, and even some blue dots. These seem like desperation plays, where the offense attempts really long passes over and over (even on fourth down) because they’re rushing to score at the end of a game.

Passing vs Rushing

We can also look at the effectiveness of different kinds of plays on 4th down. We can separate plays into either pass plays or run plays — which ones convert more often on the most exciting down in American football?

In general, it seems like run plays are better up until around 4 yards away, after which pass plays take over. The effectiveness of run plays have a tremendous negative correlation with yards needed, but pass plays only have that negative correlation until 3 yards away, after which there’s actually a slight positive correlation.

The running side is easy to explain. Since the runner carries the ball the whole time, each extra yard is a large amount of extra time that the defense gets to hone in on the only target it needs to get. This means that gaining lots of yardage by running on a fourth down play, where the defense knows exactly where the end goal of your play is, tends to be difficult. On the other hand, the consistency of run plays means that they are good at low yardage conversion.

For passing there could be a couple factors. First, the inherent inconsistency of them means that they won’t be as good on low yardage conversions. However, the fact that completed passes do often go for at least 5 yards anyway means that they are still effective if you need more yards. In fact, extending this graph beyond just 8 yards will show that successful running fourth down conversions past 8 yards pretty much never happen.

There’s also selection bias. Teams vary greatly in their confidence to complete passes on fourth downs. We can assume that the teams that elect to go for longer and longer conversions are probably more and more confident than those who don’t. Teams that are more confident of succeeding on these longer fourth downs are probably more capable of doing so consistently. The fact that the pass conversion rates stay roughly the same (even improving slightly) from 3-7 yards points to this selection bias.

Specific Plays

Bills vs Titans, Week 6

0:22 Remaining in the 4th Quarter

31-34, Titans in the lead

Possession: Bills

4th and 1 at the 3 yard line

Source: Buffalo Bills

The play was a QB Sneak from Josh Allen, in which the Bills quarterback took the snap and dove straight forward.

The stats say the decision was (probably) correct. 4th and 1 run plays have had over a 70% conversion rate this season. A successful conversion means that the Bills have a very high chance of winning the game, whereas not going for it would mean that the game would be tied and thus go into overtime. From what we’ve seen, 4th and 1s from inside the 10 yard line are usually conversion attempts, so it wasn’t like head coach Sean McDermot was going for something unusual, either. Finally, a 1 yard sneak from the 6’5’’, 237 pound Josh Allen who’s averaged over 5 yards per carry for his career seems like a no-brainer (Pro Football Reference), especially when the alternative was to basically settle for a 50-50 in overtime.

Now, there are some negatives of this play call as well. Buffalo wouldn’t be guaranteed a touchdown, even if they succeeded, because of little time remaining and only one timeout. Also, Buffalo was a middling team on fourth down. This year, they’re below average in terms of conversion tendency and at the bottom of the league in terms of conversion rate. The 70+% chance they should have had could be much lower.

In the game, Josh Allen slipped on the turf, causing him to fall (literally) just short of the first down. The Bills lost that Monday night game.

Overall, I’d still say that it’s hard to fault the Bills for this play call. It was risky, but it certainly had a good chance of working out. It would’ve been hard for anyone to predict that Josh Allen would slip like he did.

Chargers vs Chiefs, Week 3

0:48 Remaining in the 4th Quarter

24-24, tie game

Possession: Chargers

4th and 9 at the 35 yard line

Source: CBSSports

Chargers coach Brandon Staley had already decided to go for the 4th and 4. Then the Chargers got a penalty. In an incredibly gutsy move, Staley still decided to go for it on a 4th and 9. This took place in one of the other areas we examined — the 4th downs near the field goal range boundary. And, with the help of a somewhat contentious pass interference call and some more plays, it turned into a touchdown that would win the Chargers the game.

Unlike the last example, the extra 3 points added to the Chargers’ score would have also been enough to win this one. However, the value comes from being able to both keep the ball away from the Chiefs offense and get a touchdown to build enough of a lead so that Kansas City would be forced to go for a touchdown of their own. In addition, the field position means that a failed attempt wouldn’t be the end of the world (and the game would still be tied, not lost). Going for it also means the Chargers wouldn’t have to rely on their kicker to kick a 52 yard field goal, which would have been at the edge of his field goal range.

After the game, Staley mentioned the riskiness of the play but the fact that their data still said it was a positive play. He talked of the wind that was so hard for the kickers to play in (Tristan Vizciano literally missed the extra point for the Chargers immediately after their ensuing touchdown) and of the opponent factor, as the Chiefs have been known to pull off quick touchdown drives like the Chargers were worried they’d do.

Analyzing these fourth downs in such a way is likely a large part of why the Chargers have been the best 4th down team in the NFL this season (as seen by the graphs earlier). Even if it’s not so clear whether or not this was actually the right decision (I probably wouldn’t go for it), the fact that Staley was willing to consider all the factors and go for it anyway speaks to how exciting the 4th down can be.

Concluding Thoughts

In conclusion, what teams do on fourth down depends heavily on their confidence, field position, game state, difficulty of the fourth down itself, and many other factors. In general, good teams are more successful converting on these fourth downs, yet they don’t necessarily do so more frequently — this points to a difference in coaching styles. Some coaches trust their play calls when going for it on 4th down near the edge of field goal range, whereas others trust their kickers much more. These plays are often pivotal, sometimes flat out deciding who wins or loses the game. If you judge them in hindsight, the correct play will seem obvious; yet, in the heat of the moment, the amount of factors in play makes it very difficult to tell what the right decision is.



Pro Football Reference


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