Last season, NFL teams went for it on a combined 485 fourth downs, for an average of about 1.89 attempts to go for it per game.

Through Week 13 this season, there have been 378 fourth-down attempts among all teams. With every club having played 12 games now, that's an average of 1.97 per game.

That's what you'd call a slight difference. In fact, go back a decade and look at the 2008 season — there were 491 fourth-down attempts that season, or 1.92 tries per game. Want to go back even farther? Fine, then look at two decades ago. In 1998, teams (it was a 30-team league then, with no Browns or Texans) collectively went for it 455 times for an average of 1.90 times per game. That's pretty wild symmetry, in fact.

So what happened to the suggestion that teams were being more aggressive now than ever? That analytic analysis had shown coaches that they needed to go for it more than they used to in order to increase their chances to win? In fact, history is showing that coaches really haven't changed their propensity for how often they're going for it, but rather when and where to go for it within games.

But the biggest change? The success rate.

NFL teams were successfull converting fourth downs at a 46.4-percent clip last year. But this season, the improvement in converting them has been staggering — up to 56.3 percent, nearly a 10-percent improvement. Another way to think about it: Twenty-five of the 32 teams have converted 50 or more percent of their tries.

Clearly, the Philadelphia Eagles' aggressiveness was a big story when they went to the Super Bowl a year ago. Doug Pederson not only went for it the second-most times in the regular season in 2017 (26 attempts), he also had the most fourth-down conversions (17). The Eagles also were 3-for-3 in the playoffs, and 2-for-2 in the Super Bowl victory over the Patriots.

But Pederson didn't become hyper-aggressive overnight. He took a very similar approach in his first season as head coach, going for it a league-high 27 times in 2016. The biggest difference then? A lower conversion rate. The Eagles succeeded on only 13 of those, for 48.1 percent.

This season in Philadelphia has been more like 2016: They've attempted the second-most fourth-down tries (16) and converted only eight of them, for 50 percent. This year that's well below the 2018 league average, as we've shown, as the rest of the NFL seems to be putting a more concerted effort into coming into games with a clear-cut view of when they might go for it, much like teams do with two-point conversion charts and the like.

A few years ago, Sean Payton — whose team is 11-for-13 on fourth downs this season — explained his fourth-down philosophy, admitting that it's a mixture of math along with a good weathervane to know which way the wind is blowing in any given game

“Fourth downs maybe is sometimes a feel, and a lot of times it is not having a play,” Payton said. “There are some times where you have got a play you cannot wait to run, so you would be more apt to be aggressive.”

And there's a correlation to our True Turnover metric, which measures traditional turnovers (lost fumbles and interceptions) plus other lost possessions (missed field-goal attempts and fourth-down failures), and winning.

Looking strictly at the teams converting fourth downs at the best rate, the top three in the NFL right now are the Chiefs, (7-of-8), Saints (11-of-13), Chargers (5-of-6), Buccaneers (8-of-10) and Seahawks (9-of-12). Those teams have a combined record of 41-19 for a win percentage of .683.

The bottom five teams in fourth-down conversion rates: Packers (6-of-14), Browns (7-of-17), Bills (5-of-13), Raiders (4-of-13) and Cardinals (2-of-9). Those teams are a combined 17-41-2, for a 300 win percentage.

Taking this a step futher, let's look at our True Turnover table and see which teams are the most successful in terms of not only converting fourth downs but also stopping them. In the chart below you can see each team's fourth-down failures on offense and also their fourth-down stops on defense:

Looking strictly at the net gained possessions on fourth downs alone, the top teams are the Saints (plus-10), Chiefs, Chargers, Bucs and Dolphins (all at plus-5). Those teams' combined record is 40-20 for a win percentage of .666.

The worst teams in net fourth-down conversions on offense and defense are the Raiders (minus-9), Falcons (minus-8), Browns and Colts (minus-5). Their cumulative record: 16-31-1, for a .344 win percentage.

So those numbers might not be as stark as when we look directly at offensive fourth-down success rates, but they both paint a pretty clear picture. Even if teams are going for it slightly less than twice per game, those clearly are tide-shifting plays — both within games but also over the course of the season.

And that's yet another reason why we chart this stuff. Fourth downs matter.