As October approaches, several contending teams find themselves without ironclad aces at the top of their rotations. The Rangers will go into Game One with Yu Darvish, who’s riding a string of several strong starts but has struggled at times during his debut season. The A’s may have to enter October with an all-rookie rotation lacking both Brandon McCarthy and Brett Anderson. The Orioles’ rotation is fronted by Wei-Yin Chen, who’s been barely better than league average. The Cardinals are hoping the reckoning for Kyle Lohse doesn't come until 2013. Should the Dodgers claim a wild card, their hopes of advancing to the NLDS might depend on Josh Beckett. And even Yankees ace CC Sabathia has looked uncharacteristically shaky in the second half.

Meanwhile, a few other playoff locks and hopefuls can count on handing the ball to a starter who’s been consistently successful all season. The White Sox (Chris Sale and Jake Peavy), the Nationals (Gio Gonzalez and Jordan Zimmermann), the Tigers (Justin Verlander), the Reds (Johnny Cueto), and even teams on the periphery of the race like the Angels (Jered Weaver) and Rays (David Price) can rest secure in the knowledge that their top starter would match up well with any opponent in a play-in game or at the start of a series.

Aces are often viewed as integral to post-season success, but do the teams with stronger starters at the tops of their rotations really have more reason to be confident?

We can all recall teams whose playoff success (or lack thereof) supported either the Essential Ace Theory or the Extraneous Ace Theory. The Braves won the World Series only once during their streak of 14 consecutive playoff appearances, despite having three Hall of Fame-caliber starters in their rotation for the majority of their run. The Phillies won the World Series in 2008 with a rotation topped by Cole Hamels and Jamie Moyer, then got bounced in the first round three years later despite replacing Moyer with Roy Halladay, Cliff Lee, and Roy Oswalt. The 2005 White Sox won on the strength of four dependable-but-not dominant starters, all of whom made at least 32 starts but none of whom struck out as many as seven batters per nine innings. The 2002 Angels went all the way with Jarrod Washburn, who never made an All-Star team.

Obviously, having an ace is no guarantee of October success, and lacking one isn’t a post-season death sentence. But anecdotal arguments won’t get us anywhere. To come up with an answer, we have to examine how all playoff teams have fared.

To do that, BP Director of Research Colin Wyers selected the “ace” of each playoff team from the one-wild-card era of 1994-2011, defining the ace as the starter who pitched at least 120 innings with the lowest ERA. Then he came up with a normalized measure of “ace-ness,” similar to ERA+ (2-ERA/lgERA, to be precise), that allowed us to place all the aces on the same scale. Finally, he checked the correlation between the strength of each team’s ace and the difference between its winning percentages in the regular season and the postseason.

The result? A statistically insignificant .02. Park-adjusting the stats didn’t strengthen the correlation. Neither did defining “ace” as the starter with the highest WARP. Neither did running the study again using only pitchers who pitched in the playoffs, so as not to skew the results by including teams whose regular-season aces weren’t available in October. However we sliced and diced the data, we couldn’t find any evidence that the strength of a team’s top starter alone helped dictate how it would do.

October is the time when games mean the most, so it’s only natural that we would look for any signs that could help us predict which teams will still be standing at the end of the month. But searching for those signs means seeking meaning in small samples, always a risky exercise where baseball is concerned. Over the years, a number of popular theories purporting to predict playoff success have been proposed and debunked. Do teams that “back into the playoffs” after a September swoon fall to the ones with momentum? No. Are teams that rely on home runs to score at a disadvantage in October? No. Does the quality of a team’s closer, its defense, or its staff’s strikeout rate offer any hints? Not likely.

To those three suspect theories, we can add the notion that a team can’t succeed in the postseason without an ace. It certainly doesn’t hurt to have one, but we can’t count on any one player to make a major impact in a short series or a series of short series, scenarios where randomness reigns supreme. (The correlation between ace strength and regular-season success for these teams was a more meaningful .15, suggesting that having a true ace helps more over the long haul.) So what should we look at when we’re wondering which teams will succeed in the postseason, if not their top starters?

The best predictor of how often a team will win in October is, intuitively enough, how often it’s won on the way there. The correlation between regular-season winning percentage and winning percentage in the playoffs is a healthier .28, which would likely rise even higher if we looked at underlying regular-season performance rather than overall record. A great starter’s contribution is already captured in that regular-season success, and it doesn’t add any extra information to assess it in isolation. While that might disappoint those hoping to peer further into the playoff future, it’s reassuring to know that the strengths that got a team to October will continue to serve it well once it’s there.

Colin Wyers provided research assistance for this story. 

​A version of this story originally appeared on ESPN Insider Insider.

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I think you are measuring the wrong thing here. People don't necessarily think you need an elite pitcher to outperform in the post-season; they think you need a group of top 3 starting pitchers that is significantly better than your overall starting pitching for the season.
Ben didn't mention this explicitly, but I actually did look at the entirety of a team's playoff pitching staff. Again, the same lack of a significant predictor of playoff success above what regular season record tells us.
In order for this to be true, these aces must be underperforming, on average, in the playoffs. The first step would to show the effect explicitly and then if verified, speculate as to why the anomaly is occurring.
Didnt there used to be a "secret sauce" calculation that tried to measure the things that a team has that become more important in the playoffs than the regular season?
Yes--click on the linked "Not likely" in the third-to-last paragraph.
Listening to the Eff-Wild brought a statistics question to this is probably more for Colin than Ben.

Is a large number of small samples an adequate data set? For instance, 17 years of playoff series. That's 17 years of small sample sets, right? 5 or 7 game bursts that on their own are not statistically indicative or predictive, right? Does that inherent inadequacy (being a SSS thus untrustworthy) inhibit conclusions drawn from that data from being solid?

My guess is that it is an acceptable data set, but then I'm swayed the other way. If I took five games pitched from July of each Moyer, Pettite, Livan, Millwood, Suppan and Derek Lowe from each of the last 16 seasons, would I get an accurate read on their careers? I doubt it.

Hopefully, I was able to express my question properly.
I think the aggregation goes some way towards addressing the sample-size issues. I think the bigger sampling issue is the relative paucity of playoff games, not the fact that there are many pitchers taking those spots. But yes, it's possible that with a larger sample we'd be able to see the effect we're looking for more clearly.