Momentum Part 5 - Series Level Analysis

This is the final part of my series on momentum in a football game. Is momentum a causative property that a team can gain or lose, or is it only something our minds project to explain streaks of outcomes that don't alternate as much as we expect? It's been a couple months since I began this series, so as a refresher, here is what I've looked at so far:

Part 1 examined the possibility that momentum exists by measuring whether teams that obtain the ball in momentum-swinging ways go on to score more frequently than teams that obtained the ball by regular means.

Part 2 looked at whether teams that gained possession following momentous plays went on to win more often than we would otherwise expect.

Part 3 focused on drive success following a turnover on downs, which is often cited by coaches and analysts as a reason not to go by the numbers when making strategic decisions.

Part 4 applied a different method of examining momentum by using the runs test so see the degree to which team performance is streakier than random, independent trials.

In this part, I'll apply the runs test at the series level, to see if teams convert first downs (or fail to convert them) more consecutively than random independence would suggest. But first, I'll tie up some loose ends left hanging from part 4. Specifically, I'll redo the play-level runs test to eliminate potential confusion caused by a team with disparate performance from their offensive and defensive squads.

Momentum 4: How Streaky Are NFL Games?

This is the 4th part in my series on examining the concept of momentum in NFL games.The first part looked at whether teams that gained possession of the ball by momentum-swinging means went on to score more frequently than teams that gained possession by regular means. The second part of this series looked at whether teams that gained possession following momentous plays went on to win more often than we would otherwise expect. The third part focused on drive success following a turnover on downs, which is often cited by coaches and analysts as a reason not to go by the numbers when making strategic decisions.

This article will examine how 'streaky' NFL games tend to be. If momentum is real and it affects game outcomes, it would result in streaks of success and failure that are longer than we would expect by chance. But if consecutive plays are independent of previous success, the streaks of success and failure will tend to be no longer than expected by chance. This method of analysis does not rely on any particular definition of a precipitating momentum-swing, as it looks at entire games to measure whether success begets further success and whether failure leads to more failure.

For momentum to have a tangible effect on games, it does not require completely unbroken strings of successful or unsuccessful plays. But if success does enhance the chance of subsequent success, then the streaks of outcomes will be longer than if by chance alone.

For this analysis, I applied the Runs Test to the sequence of plays in a game. This produces a statistic indicating how streaky a string of results is compared to what would be expected by chance. For example, consider the following 3 strings of results of flipping a coin 8 times:

HTHTHTHT, HHHHTTTT, HTTTHTHH

The Runs Test works like this:

Momentum Part 3: After Failed 4th Down Conversion Attempts

This is the third part of my look at momentum in the NFL. The first part examined whether several momentum-swinging types of events caused any increase in a team's chances of scoring on the subsequent possession. The second part compared the expected and observed Win Probability (WP) following momentum-swinging events to find out whether those events increased a team's chances of winning beyond what we would otherwise expect.

This installment cuts to the chase. From a strategic perspective, we want to understand how momentum may or may not affect the game so that coaches can make better decisions. Often, momentum is cited as a consideration to forgo strategically optimal choices for fear of losing the emotional and psychological edge thought to comprise momentum.

Here's the thinking: If a team tries to convert on 4th down but fails or unsuccessfully tries for a two-point conversion, it gives up the momentum to the other team. The implication is that failing on 4th down means that winning is now less probable than the resulting situation indicates, beyond what the numbers say. Therefore, the WP and Expected Points (EP) models used to estimate the values of the options no longer apply. In a nutshell, the analytic models underestimate the cost of failing.

[By the same token, the reverse argument should be just as valid. Wouldn't succeeding in a momentum-swinging play mean the chances of winning are even higher than the numbers indicate? For now, I'll set the 'upside' argument aside and examine only the 'downside' claim.]

Momentum Part 2: The Effect of Momentum-Swinging Events on Game Outcomes

Recently I tried to detect the existence of momentum within an NFL game. I examined drive success based on how 'momentous' the manner in which the offense gained possession. Admittedly, that analysis only measures one aspect of momentum. In this post, I'll take the analysis a step further and look at how a team's chances of winning are affected following several momentum-swinging types of events. This approach examines the potential effect of momentum on the entire remaining part of a game, not just on the subsequent drive.

Like the previous analysis, I relied on how possession was obtained as an indication of a momentum-swing. For all drives from 1999-2013 ( through week 8), I compared a team's expected chances of winning (based on time, score, field position, down and distance) with how often that team actually won. I divided the data among three categories: possession obtained following a momentous play, possession obtained following a turnover on downs, and possession obtained following a non-momentous play.

Momentous obtainment includes fumble recoveries, interceptions, muffed punts, blocked kicks, and blocked field goals. I excluded missed field goals from the analysis because it was unclear to me how momentous they are. They are often thought of as big momentum changing events in close games but are too common (almost 20% of all kicks) to truly be momentous.

Momentum 1: Scoring Rates following 'Momentum-Swinging' Events

Momentum might be one of the most over-cited concepts in sports. It's an idea borrowed from physics, and is something we witness every day. We see it in rising tides, building storms, and boulders rolling downhill. But does such a concept apply to sports? Certainly, better teams will likely continue to prevail, and lesser teams will likely continue to lose. But that's not momentum. It's just better teams being better.

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: