Wait a Second, There May Be Such a Thing as “Momentum” in the Baseball Playoffs

The stadium scene.
Sept. 30 2013 2:06 PM

Does Momentum Matter?

Some surprising analysis of one of baseball’s hoariest clichés.

Clayton Kershaw of the Los Angeles Dodgers
Clayton Kershaw of the Los Angeles Dodgers pitches against the Colorado Rockies in a late-season matchup.

Photo by Harry How/Getty Images

This article originally appeared on Baseball Prospectus. Follow Baseball Prospectus during the playoffs for expert commentary and analysis of every series, and consider subscribing for just $39.95 per year.

Oh boy, it’s playoff time! Time for all of the baseball prognosticators out there to find that perfect little factoid that no one else has noticed about each series. It needs to be slightly surprising and counterintuitive so that the reader is entertained by your erudite knowledge of the game, not to mention your use of the word erudite. You also need to be able to make a case, probably though some questionable logic, that this factoid will, over the next five games between these two teams, not only make a difference in the outcome of the series, but will be the difference between the teams. You get bonus points if you refer to someone really obscure as an X-factor.

But if you can’t think of an X-factor that’s hipster enough, you can always fall back on the old standbys. Team A will win because they clinched early and are well-rested. Team B will win because they had to fight all the way through September to the last day and are already in playoff mode. What’s fun is that you’ll hear both explanations cited. The Braves built a 15-game lead over the Nationals in August, and it was a foregone conclusion that they would win the NL East. The AL Wild Card race, on the other hand, ended up being oodles of fun (and it’s not done yet!). We’ll find out later this week who gets to be the fourth entrant into the ALDS round. I guarantee that you’ll hear both the Braves and the Indians/Rangers/Rays picked to win (and lose) their respective series for these completely opposite reasons, the same way that the older, more experienced team and the younger, hungrier team will win.

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It’s an argument that comes up a lot. Some teams haven’t had to break a sweat yet, while others have fought tooth and nail to get to this moment. Those who argue that teams who have had to fight have an advantage are implicitly arguing for something called stress inoculation theory. The playoffs are a big stage, and you can’t blame players for being a little nervous when they step onto the field. We already know that players do show some different results than we might expect in the postseason. There’s only one October!, after all. In the same way that things that once made you nervous get less nerve-racking after you’ve done them a few times, playing big games in September makes it easier to deal with the butterflies in October.

You might call it momentum, because the teams that just squeaked into the playoffs generally played in a bunch of critical games but also won them. That’s how they got in. But really we’re arguing that this toughened them up in some way. They have already faced down the monster and won. Why not do it again? On the other side, we might cite the Beane Excremental Effectiveness Theorem. We know that anything can happen in a short series. So there’s probably already a really good chance that our momentum-having team could win the series by chance. Maybe all this talk of momentum is a bunch of excrement.

Shall we find out together?

Warning! Gory mathematical details ahead!

Let’s talk about meaningful games in September. I considered a game to be meaningful for a team if it met a few criteria

  • The team entered that day’s game(s) having not clinched a playoff spot, either division or wild card. If the team is leading its division and has clinched “the worst that can happen is a wild card” then it is considered to have clinched.
  • The team was within three games, in either direction, of a playoff spot. So, if a team was more than three games back, both in the division race and the wild card race, the game that day isn’t “meaningful.” Same goes for having a lead of more than three games. However, that can change from day to day.
  • There’s an available playoff spot that the team could still obtain (i.e., there hasn’t been a clinch on their division and the available wild cards). It’s useless to be three games back with two to play.

I ran my analyses a couple of different ways. I counted how many of each team’s last 15 games were “meaningful” and counted how many games in September (or any regular season games in early October) were also “meaningful.” The answers didn’t change. Here, I’m reporting the results for the “last 15 game” variation.

To test whether playing a lot of meaningful games in September made much of a difference, I looked at the play-by-play for all playoff series from 2003-2012. Using regular season stats, I used the log-odds ratio method to control for the quality of players on each team. This is necessary because a good way to not play a lot of meaningful September games is to win 100 games, and to do that you need some really good players. I looked at encounters in which a pitcher who faced 250 batters during the regular season was facing off against a batter with 250 plate appearances in the regular season, and calculated the expected outcome of a matchup between those two ending in several different outcomes (strikeout, walk, HBP, single, double/triple, HR or out in play.) I then added in the number of “meaningful” stretch games played by the batting team and the number played by the pitching team and loaded it into a few logistic regressions.

The answer was that there was some evidence to suggest that for the pitching team (but not the batting team), having survived a lot of should-really-win games over the last few weeks of the season predicted better results. I’m being a little bit liberal with my p-values here in some cases, but strikeouts (p = .197), walks (p = .018), HBP (p = .090), extra-base hits (p = .096), and home runs (p = .252), all registered as at least worth mentioning, and all but HBP went in the direction favoring the pitching team. P-values for crunch-time games for the batting team consistently hovered in the .80 range.

If you compare a team that had no drama coming down the stretch (zero meaningful games in its last 15) to a team that was playing for its life every night (15 such games), the effect sizes are actually on the order of a percentage point for walks and strikeouts. We would expect that the pitchers who played on the team that’s “already in playoff mode” would actually get about a percentage point more on these outcomes than we might expect based on their regular season stats and those of the batters they face. That’s a lot, something on the order of .20 or .25 in runs per nine innings based on walks and strikeouts alone. This is a fairly substantial effect.

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