Sooner or later, during a game, coaches have to make a decision on fourth down. There are many variables to consider before a coach elects whether he wants his team to go for it, kick a field goal or punt the ball away. Team's chances of successful conversion are mainly dependent on yards needed for a new set of downs. Field position is another very important thing coaches have to consider before they make a call. And although there are many no-brainer cases, there are also plenty of questionable situations. Decisions on those can eventually be the difference between winning and losing. In ideal scenario, coaches would choose the best possible option in order to maximize their team's chance of winning. Question is, what is the optimal choice on any given situation?

In his study, Brian Burke, creator of Advanced Football Analytics and currently working for ESPN, explained and presented evidence why NFL coaches should have more aggressive approach when facing fourth down decisions. In his own words, here’s how the study goes:

At each yard line, I'll calculate and compare the expected point value, based on recent historical averages, of each of the three 4th down options—punt, field goal, or go for it. The option with the highest value is the recommended choice.

As the author explains, the analysis applies to 'typical' situations only, meaning when the score is relatively close, time is not expiring and weather is not a factor. He concluded his analysis with a chart of recommended options. Here's my reproduced version of the chart from his analysis.

As we can see from the chart, NFL teams should go for it on fourth-and-short from almost anywhere on the field. The borderline between field goal attempts and punts in fourth-and-long situations is at 37-yard line.

Burke developed 4th down calculator which also takes current score and time left into account, so we don't only get expected points (EP) for one of the three possible options, but also corresponding win probability (WP). With the handy calculator I revised all fourth down decisions coach Chip Kelly faced during his head-coaching tenure in Philadelphia and compared his aggressiveness on fourth downs with previous San Francisco 49ers head coaches, namely Jim Harbaugh and Jim Tomsula.

I compared coaches' actual decisions with the recommended ones every time both, EP and WP, point to the same option. In rare instances, when EP and WP disagree and offer different option, I treated coach's actual call as the right one. One example, back in 2013 with roughly eight minutes left in the second quarter of Week 14 game against the Seahawks, down by one, the 49ers faced fourth-and-7 from the opponent's 34-yard line. Based on EP total value, the 49ers should've gone for it, but WP total suggests that the actual FG attempt was the 'right' call.

Obviously, certain situations cannot always be viewed simply as black and white and we shouldn't treat calculated recommendations as the ultimate truth. But we can use those recommendations as a guideline, or simply a tool to compare decisions by different coaches made in similar situations.

In the table above, we can see the right-call rates, or how many recommended options to go for it, kick a field goal, or punt matches with what coaches actually called, shown as a percentage of total fourth-down plays.

Chip Kelly comes out as the most aggressive decision-maker of the trio of head coaches, especially if we narrow the plays down to only fourth-and-short, with just one or two yards to go.

One can argue about how much Tomsula was really involved in play-calling and decision-making, but the fact of the matter is that both former 49ers coaches were considerably less willing to go for it on fourth down. This is particularly true if we talk about coach Harbaugh, who had to part ways with the 49ers despite his great success during four-year stay in the city by the Bay.

In her article from November of 2015, SoCaliSteph wrote about Harbaugh's unwillingness to cooperate with, and except suggestions from former team president and analytics guy, Paarag Marathe, which eventually led to breakdown of Harbaugh's relationship with the front office.

Interestingly enough, Harbaugh's fourth-down decisions during his head-coaching job with the Niners is what we would expect from an average NFL coach. In general, NFL coaches tend to be very conservative when it comes to fourth-down decisions. In regular seasons, between 1994 and 2015, NFL coaches decided to go for it on fourth-and-short on only 32.0 percent of all fourth-down plays.

Tomsula actually went for it with highest rate, with an odd 60-40 pass-run ratio, but that may be the result of rather small sample size. Kelly dialed up run play on 82.6 percent of all forth-and-short opportunities. His team converted on 73.9 percent—both are highest percentages of all three coaches in comparison.

<>If we look at only fourth-and-1 plays, an average NFL coach went for it on 42.3 percent. That is still pretty conservative, especially if we consider the fact, that teams going for it converted on 65.5 percent with more than 75 percent of those plays executed on the ground.

Coach Kelly went for it on 48.7 percent when team needed exactly one yard to keep its offense on the field. His team earned another set of downs or scored a touchdown on 73.7 percent of those plays. Redzone, one possession game, first and third quarter only, no matter how we slice the fourth-and-1 data, Kelly jumps out as the most aggressive coach. Tomsula isn't far behind and again stands out for different reason, his 49ers ran on only 57.1 percent of all fourth-and-1 plays. That doesn't fit well with what numbers otherwise suggest—NFL coaches love to call run plays in those circumstances, which is even more true for both Harbaugh and Kelly.

Let's go back to fourth-and-short situations and examine the plays Harbaugh, Tomsula and Kelly called in greater detail. In the next table, we'll be able to look at how the trio of coaches attacked those situations in terms of preferred formation, personnel and type of play.

Under Jim Harbaugh, the 49ers had one of the most diversified offenses as far as personnel groupings are concerned. Harbaugh used variety of groups to attack opposing defenses also when he decided to go for it on fourth down. Often times he would add another lineman to gain advantage up front, with quarterback most of the time under center. Two backs and two tight ends set was the most common, but it was used on only 24.1 percent of fourth-and-short plays.

In three seasons with the Eagles, Kelly preferred to run his offense from shotgun, or its hybrid version—pistol formation. The most common group he used on fourth-and-short was 12-personnel, together with 11-personnel it made for 87.0 percent of those plays. Common to all three of the coaches is that they favored run up the middle, but they did it in different ways. Harbaugh, and to some extent Tomsula, favored power runs, Kelly would do it with his quarterback handing off the ball to running back out of shotgun formation with two or even three wide receivers on the field.

Statistical studies suggest that coaches should be far more aggressive with regard to their fourth-down decisions. Burke's study is just one of many. If going for it on fourth-and-4 from midfield seems to be a bit far-stretched idea, then coaches should at least go for it more frequently on short-yardage situations, especially from opponent's half of the field.

The 49ers have a new head coach, but there's still plenty of unknowns surrounding the team, maybe most important of them all, there's yet to be determined who will be the starting quarterback come September when the new NFL season is scheduled to kick off. But one thing we can be sure of, coach Kelly will be a lot more aggressive when it comes to fourth down than most of the coaches around the league.