Momentum Is the Next Day’s Starting Pitcher

With the Dodgers nearly backing into the playoffs and the Yankees still waiting to find out whether they’ll play a streaking opponent or a slumping one in the first round, today at Baseball Prospectus I took a look at the link between late-season momentum and postseason success:

Against long odds, the final week of the 2009 regular season wound up producing down-to-the-wire excitement in both leagues, though for the most part, that excitement had nothing to do with stellar play. The Dodgers used a season-high five-game losing streak to keep the suspense regarding the NL West flag and home field advantage building for an entire week, with the Phillies and Cardinals failing to capitalize and the Rockies falling just short of overcoming a lackluster two-week stretch prior to their final sprint. Meanwhile, the AL Central has produced its second consecutive Game 163 play-in, this time due to a mad rush by the Twins and a collapse by the Tigers that may yet prove to be historic.

Against this backdrop, viewers have been treated to writers, broadcasters, and in-studio pundits admonishing such slumping teams to pull themselves together as they pontificated on the importance of heading into the playoffs with momentum. The oft-cited example remains the 2007 Rockies, who won 13 of their final 14 regularly scheduled games, then a play-in and ultimately the NL pennant. Forget the fact that just one year prior, the Cardinals dumped nine of their final 12 before becoming the team with the lowest victory total ever to win the World Series—these experts certainly did. The question obviously arises as to whether there’s truth to such conventional wisdom about whether late-season performance carries over into the playoffs. The answer is a fairly resounding no.

With the help of Eric Seidman, I pulled late-season records for every playoff team of the Wild Card era from 1995 through 2008, 112 teams in all. For each team, we recorded their record over the final seven, 14 and 21 games as well for September and whatever fragment of October remained. The results of Game 163 play-ins initially weren’t included in either the “week” records (which didn’t always coincide to weeks, but which were somewhat easier to gather) or the “month” records; including them didn’t change the results substantially. Here are the correlations between the interval’s winning percentage and first-round success:


Interval Corr162 Corr163

Final 7 .019 .016
Final 14 -.020 -.021
Final 21 -.042 -.043
Final Month -.028 -.028

That, folks, is a whole lot of nothing, an essentially random relationship between recent performance and first-round success. None of the correlations even reached .05 in either direction, and six of the eight were actually negative… Here are the correlations between those winning percentages and overall playoff success as measured by number of series won:


Interval Corr162 Corr163

Final 7 -.043 -.049
Final 14 -.097 -.101
Final 21 -.119 -.121
Final Month -.112 -.115

That’s still nothing to write home about, and the slate is now uniformly negative, suggesting that, if anything, there’s an ever-so slight inverse relationship between success in the final weeks and in the postseason. Perhaps that’s because some of these playoff-bound teams are resting their regulars more often, or simply regressing to the mean after a summer of beating up on opponents. Even if we create a points system, doubling the value of winning the League Championship Series and quadrupling that of the World Series such that the same number of points are awarded per round, the magnitude of the largest correlation—for the final month, 163-game version—still doesn’t get any bigger than .137, and it’s negative at that. It’s still essentially nothing.

From there I go on to illustrate the striking similarity in recent records between the teams that won the various rounds of the playoffs and those that didn’t; the two differ by one win over 784 games when it comes to their records in the last seven games. I also go on to cite a quick-and-dirty study I did regarding the limitations of using recent won-loss records on future won-loss records, arriving at the none-too-controversial conclusion that full-season records are better for divining the future, and that Pythagorean records, which rely upon the underlying performance, are even better for that job.

The bottom line is that as the postseason unfolds, it’s good to remember that for all the talk about momentum and its importance to a ballclub, the conventional wisdom that a team’s recent performances foreshadows their playoff fate is generally wrong. As Earl Weaver’s famous maxim states, “Momentum is the next day’s starting pitcher,” and if you’re trying to analyze what’s going to happen in a given series — which is itself a crapshoot — you’re better focusing on the strengths and weaknesses of the various matchups than on some notion of who the hotter team is at the moment.

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