The Futility Infielder

A Baseball Journal by Jay Jaffe I'm a baseball fan living in New York City. In between long tirades about the New York Yankees and the national pastime in general, I'm a graphic designer.

Wednesday, September 02, 2009

 

The Circuit Gap

In today's Baseball Prospectus/ESPN Insider double cheeseburger special, I grapple with the American League's persistent advantage over the National League in interleague play. The AL has gotten the upper hand in each of the past six years and has won at a .566 clip over the past five, or .560 over the past three.
To figure out what the strength of the two "teams" are that could produce a result where one won at a .560 clip, we turn to what Bill James called the Log5 method, one I've referenced in my articles on schedule strength and one that Clay Davenport uses — literally millions of times a day — to generate the daily Playoff Odds reports. The formula boils down to WPct = .500 + A - B, where WPct is the observed outcome percentage (.560) and A and B are the two teams. Since we also know that in this case, the winning percentages are complementary (A + B = 1.000), it's simple algebra to determine that a .530 team playing a .470 team would produce that observed .560 winning percentage.

...One revealing aspect about the AL's advantage over the NL is that even the lousier Junior Circuit teams are beating the Senior Circuit ones consistently. Sticking with the last five years of data (including this unfinished season) and splitting each league into upper and lower halves in terms of interleague records — the 35 best (or worst) team-seasons in each half in the AL, 40 in the NL — we find that AL's better half, which won at a .561 clip in those intraleague games, boosted their winning percentage to .610 in interleague games. The lower half, which produced a measly .438 winning percentage in intraleague, kicked NL tail at a .523 clip. The NL's better half posted a .551 winning percentage in intraleague play but just a .447 mark in interleague play, while the lower half dipped from .450 to .421.

This tendency persists if we break the teams into smaller groups. Here it is in quintiles:
Group   Intra   Inter
AL1 .594 .595
AL2 .550 .627
AL3 .504 .587
AL4 .458 .466
AL5 .392 .556

NL1 .580 .439
NL2 .538 .449
NL3 .503 .461
NL4 .460 .408
NL5 .418 .414
Granted, we're not talking about huge sample sizes here (14 seasons apiece in the AL groups, 16 in the NL groups), but… wow. Every NL grouping, from the best 20 percent to the worst, won significantly less than 50 percent of its games against the AL. The top three AL groupings dominated interleague play, and while the fourth AL group won less than half its games, the bottom grouping won at a robust .556 clip, thanks to a couple recent Orioles teams going 11-7, a couple of Royals teams posted winning records (including 13-5 in 2008), and just two of the 14 teams in the group finishing below .500 in interleague play.
There's no shortage of theories as to why the AL enjoys such an advantage, from the DH rule to the evolutionary pressure of keeping up with the AL East arms race to the decrepitude of the NL's worst teams to the notion that the AL somehow enjoys a market size advantage, but that's a topic for another day, and maybe another writer, as I'm mostly interested in the practical applications for this when it comes to strength of schedule and Hit List rankings.

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