I followed the American League Championship Series, between the Astros and Yankees, closely enough to be aware that the home team won every game, which almost necessarily follows from the fact that the Yankees scored 19 runs in the three games played in New York and 3 runs--yes, sic, three, 3--in the four games played in Houston. And it's not as if the ballpark in Houston is a graveyard for hitters. Being a Twins fan, I know that in the team's two championship seasons they never won a road game or lost a home game in the World Series. I therefore got curious about what the data indicate for home field advantage in major league baseball.
Turns out that over the last decade or so the home team has consistently won, year by year, from about 52 to 55 percent of regular season games. The small range contained within the lower half of the 50s suggests that the home field advantage is real but small. Six out of eleven pretty nearly pegs it, and a team that wins six out of eleven games over the course of a season will end up 88-74--probably not good enough to win the division, though a wild-card playoff berth is possible.
Does the home team's small regular season advantage get a boost in post-season games, when there is suddenly more on the line? I doubt it. Last year, for instance, there were 35 post-season games, from the two wild-card contests through seven World Series games. The home team won 17 and lost 18. So far as I can tell, this year's ALCS and the Twins' two World Series wins represent predictable statistical wanderings from the general rule holding that the home team wins just over half the time.
It's possible, I think, to give a plausible account of why, in baseball, the home team should have a small but real advantage.
The real advantage occurs in close games, when in the last innings the home team can know with less ambiguity what it has to do to win. Suppose, for example, that the score is tied after eight innings. In the top of the ninth, the visitors get their first batter to base. Should they bunt and play for one run or hit away and hope for a "big inning"? It isn't clear. But if the visitors don't score, the home team knows, when they come to bat, that scoring one run wins the game, and that moving a runner along with a bunt might therefore be a good strategy. On the other hand, if the visitors score twice, and the home team gets their first batter to base in the bottom of the ninth, they know not to give away an out in order to advance the runner, since they need two runs to avoid losing. The home team, in other words, can adopt the maximal strategy. There isn't anything like this in any other sport that I know about.
That the home team's advantage is small may be explained by another feature unique to baseball--the place of the starting pitcher, which changes from game to game. Sometimes you hear people say things like "pitching is 60% of baseball," which sounds like a lot, but every fan has been to games in which 100% is more like it: one team's pitcher can't be hit, and the other team's can, so guess who wins? Over the long haul, the pitching advantage belongs to the visitors as frequently as it does to the home team, and this holds down the home team's winning percentage--to no higher than about 0.53, it seems.
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