The myth of second-half momentum

Back at the end of July, there was one overarching narrative: the Tigers and A’s engaging in an arms race to try to get an advantage for their inevitable October matchup. On the day the Tigers acquired David Price and the A’s acquired Jon Lester, Oakland had the best record in baseball, while Detroit had a comfortable four-game lead in the AL Central; since adding their respective aces, both have surrendered their divisional leads, with the A’s falling so far behind the Angels that their chances of winning the AL West now stand at a fraction of one percent.

Toss in the quickly fading Milwaukee Brewers — 60-49 on July 31, 14-22 since — and we have three strong first-half contenders limping to the finish line. Like the A’s, the Brewers should basically punt any hopes they still hold about a division title, and are now hoping entirely for a surge that will put them back in the wild-card race. But even if the A’s, Tigers and Brewers manage to right their ships, should they really be optimistic about their chances in October? After all, they’ve spent the better part of the past few months playing lousy baseball, and at some point, doesn’t a second-half slump become an indicator of underlying problems that would derail a playoff run even if the team managed to get to the postseason? 

In a word: no.

It might feel like a team’s recent performance should predict October success, but history just doesn’t support the concept of second-half momentum carrying over into the postseason. To see if it did, I pulled the second-half records of every team that made the postseason between 1995 and 2013 — there were 156 such teams — and ran a correlation between their second-half winning percentage and their postseason winning percentage. The correlation between the two numbers was 0.09, meaning that there is almost no relationship between a team’s second-half winning percentage and its record in October, assuming it plays well enough to still get into the playoffs, of course. 

We can even look at the extreme ends and see that the lack of a correlation isn’t simply due to a bunch of teams in the middle of the pack mucking things up.  Here are the top five and bottom five playoff teams by second-half winning percentage over the 1995-2013 timeframe.

The five best second-half records:

Second-half success

Team 2nd Half Win% Postseason Win%
2001 OAK 0.773 0.400
2002 OAK 0.716 0.400
1999 ARI 0.712 0.250
2001 SEA 0.707 0.400
2009 NYY 0.703 0.733
Average 0.722 0.513

The five worst second-half records:

Second-half slumps

Team 2nd Half Win% Postseason Win%
2005 SDP 0.466 0.000
2006 STL 0.473 0.688
2006 DET 0.486 0.615
1998 CLE 0.506 0.500
1995 COL 0.507 0.250
Average 0.475 0.543

The five teams that stumbled into the playoffs with the most meager second-half records combined to go 25-21 in the postseason; the five teams that played like the ’27 Yankees for the final three months of the year went 20-19. Both groups had one team go on to win the World Series — the 2006 Cardinals and the 2009 Yankees — but the losing group actually had one more team make it to the League Championship Series, as the Tigers were the team beaten by the Cardinals in the 2006 fall classic. And it’s not like the Tigers limped there, as they went 7-1 against the Yankees and A’s on their way to the AL pennant. 

Don’t like total sample correlations or five-game mini-samples? We can even break down the 156 teams into quartiles, creating four groups of 39 teams each based on their second-half records. The 39 teams that dominated the second half of the season — winning at a .663 clip along the way — managed to go 160-160 in the playoffs. The 39 teams that posted the worst records in the second half of the season — they still managed to post a .532 record, since it’s tough to lose for half a season and still make the playoffs — went 146-145 in October. 

The teams with a better second-half record did manage to play a little deeper into October on average, but the difference amounts to an extra half-game per team for the top quartile versus the bottom quartile. And we should expect to find some advantage for teams with better second-half records, since that sample should include better overall teams than the ones that struggled down the stretch. Still, even with that selection in bias in place, we find basically no real postseason advantage for teams that finished strong versus those that played poorly. 

A second-half record doesn’t really tell us anything about what a team should expect to do in the postseason. And no, a team’s September record or even a team’s performance over the final few weeks of the season don’t really matter, either, at least when it comes to predicting postseason success.  

If anything, the A’s, Tigers, and Brewers are examples of why we shouldn’t put any stock in mythical momentum. After all, who had more momentum than the A’s going into August? Not only were they baseball’s best team by nearly any measure you wanted to use, they added two of the best pitchers on the market, plus a third quality innings eater for extra depth. They took a very good team, made it even better, and almost immediately stopped winning for no obvious reason.

That’s the thing with streaks; we can’t know when they start or when they end until after the fact. For all we know, the A’s struggles ended with Tuesday night’s drubbing of the White Sox. And in a few weeks, this might all seem irrelevant to Oakland’s situation if it uses that as the start of a long winning streak. The A’s probably won’t, but then again, probability didn’t think they’d hand the Angels the division before we even reached the second week of September either. 

So fans of the A’s, Tigers and Brewers, take heart. These slumps have not been fun to endure, and they’ve made your road to the World Series more difficult, but they aren’t signs of impending doom. If these teams can turn things around and get themselves into the playoffs, their second-half struggles should be forgotten. Beyond making the road to the postseason harder, these slumps don’t really matter much when October rolls around.