Does League Superiority Exist?

Posted by Bill Baer on Tuesday, September 15, 2009 at 7:03 pm

With a thirteenth season of interleague play recently finished up when the Chicago White Sox defeated the crosstown rival Cubs in a make-up game, it marked the sixth straight season in which the American League has out-dueled its National League counterparts. Over those last six seasons, the AL is 840-671, a winning percentage of .556.

The AL’s dominance over the NL is now essentially a baseball truism, backed up by the fact that the NL hasn’t won an All-Star Game since 1996 when it was held at the now-demolished Veterans Stadium in Philadelphia. To give you an idea of how long ago that was, only three of the 18 starters on both league’s starting rosters are still active: Ivan Rodriguez, Chipper Jones, and John Smoltz.

Yet the AL’s dominance doesn’t show up in World Series results. The 2004 Red Sox and ‘05 White Sox were the last back-to-back winners by a league and the last league to three-peat was when the Yankees won back-to-back-to-back from 1998-2000. Is the claim that the AL is superior false? We can examine it in a couple ways.

First, I threw the interleague records into Excel and ran a simple t-test. The overall population mean (1997-2009) for AL wins is 129 with a standard deviation of 16. The sample mean (2004-09) is 140. And, of course, we have 13 samples, one for every year of interleague play. A t-test gives us a z-score of 2.48, which is lower than the 99% confidence interval at 2.58, so we conclude that the AL’s dominance since 2004 is not statistically significant.

Next, I put the average OPS at each position by league into Excel, and made some comparisons. The following table shows the difference in OPS at each position. Red means that the AL had the higher OPS by that amount, and green means that the NL had the higher OPS.

We’re seeing a lot of green here, aren’t we?

Pitching can’t really be compared since NL pitchers collectively accrue close to 6,000 plate appearances per season, which helps an NL pitcher’s OPS-against by around .020.

Any dominance that the AL does have over the NL can likely be simply attributed to random chance, and aside from that, money. The average MLB team payroll is $88.5 million. The AL, with a lot of help from the Yankees, has an average team payroll of over $93 million while the NL has an average payroll of over $84 million. The difference, about $9 million, or about the value of Adam Dunn’s production this year according to FanGraphs.

It’s not all that much, and that’s not to denigrate Mr. Dunn. However, generally speaking, teams do get more production out of spending more money. The NL has the four smallest payrolls belonging to the Washington Nationals, Pittsburgh Pirates, San Diego Padres, and Florida Marlins, who are #27-30 respectively. However, they have not been the biggest contributors to the NL’s lack of success in interleague play, as the Nationals, Pirates, Padres, and Marlins went a combined six games under .500 (30-36, .455) in interleague play this year.

Should the AL win the World Series this year and should the AL win the All-Star Game next year and should the AL win interleague play next year, it still would not point to dominance over the NL. Sometimes, when you flip a fair coin, it lands tails seven times in a row. Some statistically improbable stuff happens simply out of randomness.

The defending World Series champion Philadelphia Phillies went 6-12 in interleague play this season. Three of the twelve losses (25%) came in a series sweep at the hands of the Baltimore Orioles who currently have a .408 winning percentage overall. When you only play one or two three-game series once a year, stuff like that is going to happen. No one will argue that the series reflected both teams’ true talent levels.

On a broader note, the comparisons between the leagues are especially meaningless in this day and age, as players switch teams like politicians switch positions on issues. The only reason the leagues have any relevance is due to the playoff system. If instead the top eight teams with the best records made the post-season, there would be no need for an American or National League. Both leagues were separated to begin with because they were initially separate entities not under one roof.

The AL is not a vastly superior league to the NL and such comparisons should cease being made as the MLB becomes more and more colorless, for lack of a better word.