As I watched the Ravens survive a dramatic 49ers comeback attempt in Super Bowl XLVII, all I could think about was Chip Kelly.
As a lifelong Philadelphia Eagles fan—and a believer in basic statistics for almost nearly as long—I can’t help but get excited about the edge my Eagles will gain from Chip Kelly’s decision making.
I was so busy with Stanford football this fall that I found myself lacking the motivation to procrastinate my homework another four hours to watch the pathetic Eagles–or any other NFL teams for that matter—very often.
Maybe that’s why I was so stunned to see how poor NFL coaches’ decision making was on fourth down in the playoffs. I had gotten used to the college game, where inconsistent kicking games have helped lead coaches to better decision making. Chip Kelly has been at the forefront of college head coaches in this category.
I had always heard that coaches were too conservative, but I never fully understood the argument until I took Mathematics of Sports (STATS 50) last fall. Out of all that we learned, the one thing that has stuck closest to me is how to calculate the value of field position in the NFL.
I’ll keep the math as simple as I can make it. Let’s call V(x) the value in points of a first down and 10 at the x yard line—note that for the formula, the opponent’s 30-yard line would effectively become the 70-yard line. My professor showed that we can do a pretty good linear approximation for the formula, where V(x) = -2 + .09x.
Though both Harbaughs made some questionable decisions in the Super Bowl, I’m going to focus on one specific decision by San Francisco head coach Jim Harbaugh. Late in the third quarter, he made a decision that clearly shows how clueless NFL coaches are when it comes to fourth-down choices.
After a penalty for running into the kicker gave the Niners a fourth down and two at the Ravens 16-yard line, Jim Harbaugh had a decision to make. His kicker, David Akers, who had been spotty at best all season, had just missed a 39-yard field goal attempt. However, the penalty gave him a second chance.
Should Harbaugh have let Akers kick it again or should he have sent the offense back on the field? Let’s take a look at the math.
Based on the model introduced earlier—V(x) = -2 + .09x—if the 49ers failed on fourth down, the expected value of the Ravens’ next possession would be around -0.7 points. This is equivalent to 0.7 points for the 49ers—due to poor field position for the Ravens. If the 49ers converted, the expected value of the rest of the possession would be a minimum of 5.7 points—I say minimum because that assumes the Niners got just enough yardage on fourth down to get the first.
Now what do you think the chances are that the 49ers convert on the play? According to The New York Time’s sports blog The Fifth Down, the historical success rate for the offense on fourth and two is 60 percent. I’ll spare you the rest of the math and just say that, based on that number, the 49ers’ expected value by going for it would have been 3.7 points, well above the total of three points for the field goal.
I’m sure some of you will argue that the Ravens’ defense is better than average, therefore decreasing the chances that the Niners would convert on the play. However, for the math to even suggest that Harbaugh should have let Akers kick doesn’t start to swing until you put the conversion odds at below 50 percent; there’s no way the Baltimore defense has that big of an advantage over the potent San Francisco offense.
I know this is only one decision, but I think it is emblematic of a trend that dominates the NFL. Well, a trend that dominated the NFL.
Chip Kelly would have gone for it—and if he had Marcus Mariota, Kenjon Barner and De’Anthony Thomas, he probably would have made it.
These single plays, by themselves, don’t decide whether a team is going to win the Super Bowl. However, over the course of the season, making the right decision on fourth down can be very significant.
I’d bet there is at least a play or two in every game when most NFL coaches throw away one point of expected value. It might not sound like much, but in the hypercompetitive NFL, these plays could make the difference between making and missing the playoffs.
I’m glad Chip Kelly will be the one making these decisions for the Eagles.
Now that Chip Kelly has left Oregon, Sam is considering trading in his David Shaw sweatshirt for a Chip Kelly visor. Give him fashion advice at email@example.com and on twitter at @samfisher908.