Quantifying Momentum in the NFL
Photo Credit: Kyle Terada/USA TODAY Sports
Sunday’s NFC championship game between the Detroit Lions and the San Francisco 49ers was a roller coaster ride. The Lions seized a 17 point lead by halftime, only to see the 49ers storm back and win the game 34-31. In the second half, the Lions turned the ball over on downs twice after going for 4th and 2 at the SF 28 yard line and 4th and 3 at the SF 30 yard line. Each of those failed plays appeared to energize the Santa Clara crowd and help propel the home team to the Super Bowl.
Watching the game had me thinking: can we quantify momentum in the NFL?
In this analysis, I’ll look at all touchdowns and field goals over the past 24 seasons to see whether the scoring team, on average, has outperformed its post-score win probability.
At first glance, the momentum seems subtle, as scoring teams exhibit an average win probability of 61.51% after scoring, but in reality, they end up winning at a slightly higher rate of 61.74%, resulting in a momentum premium of 0.23%. Upon closer examination, the entirety of this momentum premium exists when the road team scores. Home teams, on average, have a win probability of 67.14% post-score and ultimately secure victory at a rate of 67.18%, resulting in a marginal momentum premium of 0.04%. Conversely, road teams have a win probability of 55.34% post-score and manage to secure victories 55.78% of the time, resulting in a more substantial momentum premium of 0.44%.
Let’s see how the road momentum premium trends from kickoff until the end of the game, depicted in Figure 1 below.
Figure 1: Win probability post-score, actual win%, and Road Momentum Premium after each touchdown and field goal for the past 24 seasons of NFL games. X-axis is time elapsed in the game.
As seen in Figure 1 above, road momentum premium has higher volatility in the first 17 minutes of the game before settling in closely to its true value of 0.44%.
Explaining Road Momentum Premium
The best explanation for why scoring momentum exists for the road team after the first 17 minutes of play is the impact the score has on the home crowd. The road team has the opportunity to reduce the energy in the stadium and thus disrupt its opponent’s home field advantage with a score. Taking the home crowd out of the game in the first 17 minutes is much more difficult with so much time left in the contest, but a score later than that can cause some nervous feelings at the stadium.
Using Road Momentum Premium Going Forward
How should this analysis impact the decision making of NFL teams?
After the first 17 minutes, and on the road: if a close to break-even win probability decision between kicking a field goal and going for fourth down presents itself, kick the field goal.
After the first 17 minutes, and on the road: if a close to break-even win probability decision between punting and going for fourth down presents itself, go for fourth down.
