The Inequities of Interleague

Well, interleague play is over for another season. This season the two leagues played 252 games against one another with the AL taking a decisive 154 games for a hefty winning percentage of .611. The AL has also won the last 10 All-Star games and six of the last eight World Series. A lot has been made about the big difference between the leagues this season and I even caught this quote from USA Today last Friday:

"It's enough to make AL general managers take a harder look at NL pitchers at the trade deadline. Just because a pitcher is having success in the NL doesn't mean it will translate into the same performance in the American League.

'For the first time,' [Billy] Beane said, 'we're going to have to take a harder look at that. But at the same time, I don't want to get carried away. The Mets are still a good club, no matter what league they play in. And there's no question in my mind the Cardinals would be winning over here too. It's just a bad time to be in the American League these days.'"

What's interesting of course is that Beane is intimating that perhaps in the future they'll be applying league adjustments to project NL players when they move into the AL and vice versa. That's the sort of thing that performance analysts haven't done at the major league level but of course are familiar with when translating statistics between say, the Federal League and the AL or AAA and the majors. Clay Davenport's Davenport Translations (DTs) apply these kinds of adjustments.

But the question is whether the AL is really the stronger league and if so by how much? First, I should mention that since interleague play was initiated the NL had the advantage pre-2006 at 1,104-1,096 and now the AL has taken the edge at 1,250-1,202. As a result, if there is a strength advantage for the AL, that advantage hasn't manifested itself until recently. Last season they went 136-106 and so now are 290-204 over the past two seasons. Second, there are different views about how to try and measure the strengths of various leagues. Obviously setting a baseline to use a measuring stick is the most apparent one. Mitchel Lichtman this morning started a two-part series on The Hardball Times that explores this issue and so you'll want to stay tuned for part 2 tomorrow. I can say that in a preliminary analysis Clay Davenport's technique that attempts to measure league differences hasn't detected a difference for 2006. Jim Baker also takes the position of many that it's probably too early to tell and that we'll need a few years of data to make more definitive statements either way.

Additionally, I found it interesting that the AL also overshot it's pythagorean record against the NL in 2006. This year the AL scored 1,336 runs and the NL 1,115. That works out to a projected record of 148-104 for the AL, six games worse than their actual record. That's still a pretty hefty advantage however. It turns out the Rockies were the best NL team in interleague play going 11-4 while the Giants were second at 8-7 making them the only two NL teams with winning records. One of the points that Lichtman makes in his article this morning which I found interesting is that despite the AL having the advantage offensively by having a player who is naturally a DH (NL DH's do worse than their AL counterparts in interleague games), NL pitchers make up some of the difference and in the end that difference accounts for only about a half a win per year.

On the attendance front interleague games drew an average of 34,097 fans which eclipsed the record of 33,703 set in 2001. Overall interleague games drew 15.5% more fans than other games which averaged 29,520 fans thus far. Since 1997 interleague games have drawn 13.2% more fans than other games. And that means it's hear for the forseeable future.

Not surprisingly Joe Mauer hit .492 (30-61) in interleague games to lead everyone and David Ortiz hit 9 homeruns with Ryan Howard second with 8. Francisco Liriano went 5-0 while Johann Santana had a 0.82 ERA.

Update: Lichtman has published his second article on AL and NL differences over at THT.

Update: Part three is now available.