In this article, I'll explain why I think we see momentum when it's not really there. And to test the existence of momentum within NFL games, I'll compare the results of drives following 'momentum-swinging' events with those following non-momentum-swinging events.
For momentum to be a real thing in sports, it needs to have some connection to reality beyond the metaphysical and metaphorical. The theory is that good outcomes are emotionally uplifting, which in turn leads to better performance, which then feeds upon itself. It's understandable to believe in game momentum when we see games like this each week:
I think the idea of momentum in a game is almost fully attributable to an illusion. It is well known that people are not good at interpreting randomness. True independent random events tend to be less alternating and more streaky than we expect. I've previously written about the story of a professor who challenges half his class to invent a random sequence of heads and tails and write it on the chalkboard. He tells the other half to actually flip a penny and record the results on another chalkboard. The professor excuses himself during the process. After the class is done, he re-enters the classroom and can instantly tell the true random sequence from the fake sequence. The real one has many more long runs of the same result than the fake one, because the students underestimate how streaky real randomness can be.
It's no different for fans, coaches, players, and commentators. Naturally random performance on the field often appears as streaks, and we're interpreting this as momentum because it violates our expectations of alternation. And if we allow ourselves to cherry pick the endpoints of our definition of when a run of momentum begins and ends, the illusion of momentum is enhanced.
I suspect our false expectations of alternating outcomes in a random system probably come from nature. As I wrote before last year's Super Bowl:
...I think fans and analysts alike are susceptible to the idea of momentum because our brains are geared to detect patterns from nature. Football teams and their win-loss records are very, very abstract constructions, but our brains aren’t built for such abstractions.
A boulder rolls down a hill and gains momentum. A spark sets a fire, and soon it has built into a blaze. The rains come and soon the river is rushing over its banks. Momentum is everywhere in nature, but applying it to abstractions like team win-loss records in a relatively small sample of football games is what I call voodoo analysis.
Voodoo analysis is the application of apparently intuitive patterns beyond their natural settings. A football team is not a boulder rolling down a hill. It’s not a river bursting through a damn. It’s not a spreading fire. Our brains are continuously looking for patterns like these, and often see them even when they’re not there.
One way to test momentum is to examine drive outcomes based on previous outcomes. If momentum exists in football, we should be able to detect increased performance following a prior momentum-changing event. After a significant event such as an interception, fumble recovery or blocked kick, we would expect to see an inspired team that succeeds more often than we would expect if such a momentous event had not occurred.
Looking at drives from the 1999 season through week 8 of the 2013 season, we can get an idea of whether momentous events tend to spur increased performance. For this analysis, I examined scoring rates on drives according to how possession was obtained. Drives are classified as "Momentous" and "Non-Momentous." Momentous obtainment included turnovers, blocked punts, blocked kicks, muffed punts, muffed kicks, and of special note--turnover on downs. Non-momentous obtainment included standard punts and kickoffs. I was unsure how to classify missed FGs. They can be thought of as momentum-swinging in close games, but they occur on 20% of all attempts, too common to consider momentous.
There is more to momentum that just how possession is obtained, but there are few events that cause bigger impacts on Win Probability than turnovers, blocks, and stops on downs. Although we may not be capturing all momentum-swinging events, we are capturing the vast majority of them. Not every drive would be expected to benefit from a momentum effect following a big play, but some of them would, and certainly we would not expect performance to be depressed.
I excluded all 4th quarter drives to avoid known bias. Late in games, teams in desperation situations will often take more risk than otherwise warranted, causing a turnover or other event that favors the opponent. That opponent may not need to score to seal the win, which would suppress their scoring rates. I also excluded all drives that ended with the expiration of the half.
Of course, where on the field the ball is obtained will have the largest effect on the likelihood of scoring, with or without momentum. To account for this effect, I broke out where drives started in 10-yd chunks of the field.
Here are the results. The first chart below plots the proportion of drives that result in TDs according to where on the field the drive began. The three lines represent how possession was obtained. The blue line is for momentous obtainment. The red line is for non-momentous obtainment. And the green line is for drives that began following a turnover on downs. If momentum has a real effect, we would expect the blue and green lines to be above the red line throughout the field.
TDs aren't the only way to score, so here is the same plot of the proportion of drives that result in a FG. Note that the sample size of 'turnover on downs' cases is very low deep in a team's own side of the field, for logical reasons. Of the 998 drives obtained on downs, only 32 were obtained inside the 30 yard line.
To measure the combined scoring success for TDs and FGs, I computed the average points scored per drive--7 points for a TD, 3 for a FG, and -2 for a safety. Note this is a much simpler metric than the Expected Points concept, which measures net point potential. Just as above, if momentous events lead to increased subsequent performance, we would expect to see increased scoring rates on drives following those momentum-swinging events.
It appears, possession gained by momentum-swinging means does not produce any additional chance of scoring on the subsequent drive. If anything, it appears to slightly decrease. And possession gained by a turnover on downs results in fewer points scored by the team that made the stop.
This analysis covers only part of the definition of momentum. For example, it does not examine the effect of big offensive plays or kick returns. However, it does examine the majority of events commonly thought of as momentum-swinging. It also ignores events after the drive immediately following the momentum-swinging event. But I think everyone would agree that the notion that momentum is somehow real but has no effect on the most proximate subsequent events is implausible.
I'll take a look at other measures of momentum in future articles. The second part of this series uses the Win Probability model to see if teams win more often following a momentum-swinging play than the model would expect. In the meantime, here are some previous looks at the concept.
Big Plays and Psychological Momentum in the NFL
Interceptions and Counter-Momentum
Football Freakonomics - Is Momentum a Myth?
Searching for Momentum in the NFL
Is Momentum Descriptive or Predictive?
Football and Philosophy - A Model for Understanding Momentum
Momentum and Organizational Risk Taking
A Multidimensional Model of Momentum in Sports
Nomentum (part 1), part 2 (Excellent qualitative discussion of the problems with momentum theory in football)