One of our favorite websites, Football Outsiders, recently released their Aggressiveness Index for the 2014 season. They have been doing this for the past ten seasons, and refined their grading system along the way.
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 in equivalent situations. 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. AI was expanded two years ago to include plays when the offense is on its own side of the field, excluding those obvious catch-up situations. A slightly newer version of AI we are using now also adjusts to judge coaches on all fourth-and-short opportunities, even when the 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.
Based on their metrics, Sean Payton was the most aggressive play-caller last season, followed by Marc Trestman at #2 and Jim Caldwell at #3.
What about Trestman and Caldwell, this year's other leaders in fourth-down aggressiveness? Trestman's aggressive playcalling was probably highlighted in the Week 11 win over Minnesota. Near the end of the third quarter, a missed field goal by Blair Walsh left the score 14-10 Chicago and gave the ball to the Bears on their own 28. On the ensuing drive, Trestman twice decided to hand the ball to Matt Forte on fourth-and-1 instead of kicking a field goal to make the score 17-10, first on the Minnesota 27 and then again on the Minnesota 6. Forte converted both times and the Bears ended the drive with a touchdown that made the score 21-10. (They won 21-13.)
Chicago Bears fans will remember that Trestman's approach from Day 1 was to take a metrics-driven approach to his play-calling... he wanted to ensure that plays were being called according to the probability of success based on that specific scenario in the game.
Often times, Trestman did appear very aggressive, but he was also just as often criticized for his play selection.
On a different note, new Chicago Bears head coach John Fox was ranked 14th last year in the Aggressiveness Index while with the Denver Broncos.