Baseball's World Series is played over the best of seven games. The first

two games are played at the home field of one team (we will call

this one team A), the next three at the home field of team

B, and the last two at the home field of team A. Given that

teams are more likely to win games on their home fields, does

this give team A an advantage?

Keith Burgess-Jackson href="http://analphilosopher.blogspot.com/2004/07/myth-of-home-field-advantage-if-youre.html">argues that neither team has an advantage:

Every Series goes either four, five, six, or seven games. We

don't know at the outset how many games it will

go. Suppose it goes four games. Then there will have been two

games in each team's park. No advantage. Suppose it goes

five games. Then there will have been three games in team

B's park and two in team A's park. Advantage to team

B. Suppose it goes six games. Then there will have been three

games in each team's park. No advantage. Suppose it goes

seven games. Then there will have been four games in team

A's park and three in team B's park. Advantage to

team A.Let's take stock. In two of the scenarios, there is no home-field

advantage. In one scenario, the team that begins at home has an

advantage. In one scenario, the team that begins on the road has

an advantage. It's a wash! Where's the overall

advantage?

Is he right, or does team A have the advantage? The answer is below the fold.

Team A has the advantage. The easiest way to see this is to imagine

what would happen if all seven games were always played instead

of stopping when one team gets four wins. Then team A would

have the advantage since it plays one more game at home. But

once a team reaches four wins, playing the remaining games makes

no difference to which teams wins the Series, so team A has the

same chance of victory in the "stop at four" and in the "play all seven"

formats. So team A has the advantage in the "stop at four"

format that is actually used.

A second way to see it is to consider what would happen if home field

advantage was absolute. Then each team wins all their home

games and team A wins the Series in game 7. So team A has

the advantage again.

For people who are not convinced, I have calculated the actual

probabilities in the table below (this requires that your browser

supports Javascript). The home team has won 57% of the games in

World Series play^{1}, so there is a 57% chance the Series score will be

1-0 after game one and a 43% chance it will be 0-1. The score will be

1-1 after two games if either B wins with the score 1-0 (43% of

57%) or if A wins with the score 0-1 (57% of 43%). The total is

49% (43% of 57% + 57% of 43%). Similarly you can work out the

probability of each Series score and add up the ones where A

wins to work the chance of A winning the Series and see that A

has a 52% chance of winning the seven game Series. The

calculations assume that the results of the games are

independent---that there is no such thing as "momentum" where

the winner of a game is more likely to win the next game.

You can experiment by entering different percentages for home field

advantage or different formats for the sequence of home and away

games. (Press 'Enter' after changing values to have the table

update.) Notice that the format makes no difference to A's

chance of winning the Series---it's the same with BBBAAAA as it

is with AAAABBB.

Now there are some possible explanations for the extra four-game Series.

For example, I assumed that the teams are evenly matched except for

home field advantage, but if one

team is better than the other, that increases the chance of a four

game Series. (Try entering 67 and 47 as the percentages that A and B

win at home.) Trouble is, that *decreases* the chance of a

seven game Series. (Try it---the *p* value even goes down.)

Similarly, if there is "momentum" and winning one game makes a team

more likely to win the next game, that makes four-game Series more

likely but seven-game Series *less* likely.

The only way that there can be a sixth game is if the score is 3-2

after five games. For the Series to go on to seven games, the team

that is behind must win the sixth game. Remarkably, that has happened

31 out of 49 times or 63% of the time instead of the 50% you would

expect. I can't think of a good reason why this would

happen. If you can, leave a comment.

**Update 30 Oct:** Included result of 2004 Series.

^{1} The results of all the

games are available from
href="http://baseball-almanac.com/ws/wsmenu.shtml">this site.

I collected the results for the 78 world Series that used the AABBBAA

format for games (the Series from 1924 to 2004 except for 1943 and

1945) and put them in this file so you do your own

calculations if you are so inclined.

^{2}

Alan Abramowitz has
href="http://www.emory.edu/EMORY_REPORT/erarchive/2003/October/October20/10_20_03firstperson.html">article

on home field advantage. While he finds that the team with home field

advantage

wins more often, he argues that this is not because they get to play

the seventh game at home:

playing game seven at home does not appear to be a significant

advantage in the World Series. ... Moreover, since the 2-3-2 format was

introduced in 1924,

the home team has won only 16 of 31 seventh games (52 percent), far

below the 57 percent success rate of the home team in all World Series

games.

I don't think 52% is "far below" 57%. In fact, if the home team had

won just two more of those seventh games the success rate would have

*more* than 57%. If you do a statistical test (the Fisher

Exact Test) you will find that the difference between 52% and 57%

is not even close to being statistically significant, so it is wrong

for Abramowitz to reject the notion that the advantage comes from the

extra home game. (He also miscounts the number of wins for the home

team in game seven---it is actually 17 of 32, or 53%.)

Burgess-Jackson's argument is an interesting illustration of the pitfalls of the principle of insufficient reason. He implicitly assumes that seven-game and five-game series are equally likely (actually, if this were true, the setup would be biased in favor of B since a 3-2 edge in home games is better than 4-3).

Given equally matched teams, there's at least a 35 per cent probability that the first six games will split 3-3 (this probability is greater, the greater the home team advantage).

The probability of a 4-1 series is about 20 per cent. I think this decreases with home team advantage, but I'm not sure.

This was really interesting, especially your game 6 data. That prompted me to break it down a little more. What I seemed to see is that the HFA is at its strongest when a team behind in the series returns to its home stadium (i.e. in games 3 and 6). The URL I entered goes the the BaseballThinkFactory discussion of your article, where I posted my data. You should take a look; I think you'd find it interesting.

Great job, this was fun to look at.

The biggest question is if the change of the rules as regards the DH affects series probability significantly.

While a seven game series is more likely than a five game series, that isn't the main error in Burgess-Jackson's argument. Suppose that the maximum number of games was six (so if it ends 3-3 the title is shared). Then B-J's argument would lead you to conclude that team B has the advantage because B gets home-field advantage in a five game series and and neither team gets the advantage in four and six game series. But that conclusion is incorrect. B is more likely to win in a five game series than A (14% to 11%), but A is more likely to win in a six game series than B by exactly the same margin (17% to 14%). So, actually neither team gets an advantage in this best-of-six format. The reason is the same as the one I gave earlier -- if you always played six games the same team wins as when you stop at four wins, and neither team has home-field advantage after six games.

You can experiment with formats like BBBAAAA and AAAABBB and you will see that it makes no difference to the probability that A wins.

Of course, the team that wins game 1 wins the Series 60 some odd percent of the time, so Burgess-Jackson's argument falls apart with just that statistic. That means the team playing at home in Game 1 has an empirically demonstrated advantage. He'd know that if he'd watched Game 1 of this year's series (though his reasoning capabilities may not be sufficient to put 2 and 2 together there).

One potential reason why the trailing team wins more than their share of Game 6's is that managers adopt different strategies. The trailing manager is desperate and may bring in Game 7 starter or overuse their closer or play an injured player. One way to test this would be to see whether the Game 6 winner lost more than their share of Game 7s (controlling for home field advantage. A second (partial) reason could be home field advantage. What fraction of Game 6's is the trailing team the home team? There also may be a psychological factor (back to the wall, but still hope, whereas down 3-0, no hope (unless you're the Red Sox)).

Marc, an interesting idea. Of the 32 7-game series, 18 were won by the winner of game six, so the game six winner won more than their share. When the trailing team was playing at home, they won 21 out of 28 game sixes, which is way more than you would expect from home field advantage.

With all of the talk about Game 5 and Game 6, I'd like to point out that this series only went four games. Of course this doesn't bear on your very interesting discussion, but I've been waiting for decades to gloat a little.

Another factor worthy of consideration in the results of games 6 and 7: perhaps home field is more likely to decide the result when the teams are closely matched than when they are not. Think of it not as a co-factor but a tiebreaker. Thus, the more likely a series is extended, the more likely it's decided by the home field. How to measure this? Well, were series of more than 5 games demonstrative of a larger home field advantage?

What if we had 3-3, then the seventh game (if there is one), played at a field where neither team has the HFA? Kinda like always playing the FA Cup Final at Wembley.