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World Series (Posted on 2002-10-30) Difficulty: 3 of 5
What is the probability of a world series ending after 4 games? After 5 games? 6 games? 7 games?

Assume that each team has equal probability of winning each game, regardless of who has won the previous games.

(For the baseball-challenged, the World Series ends after one team has won 4 games. Ties are not possible.)

  Submitted by Jim Lyon    
Rating: 3.5714 (7 votes)
Solution: (Hide)
In order to have a 4-game series, games 2..4 must end the same way as game 1. p = 1/8

In order to have a 5-game series, games 1..4 must be split 3-1, then game 5 must be won by the leader. The probability of splitting 3-1 is 4 * 1/16 * 2 = 1/2. So overall, for a 5-game series, p = 1/4.

In order to have a 6-game series, games 1..5 must be split 3-2, then game 6 must be won by the leader. The probability of splitting 3-2 is 10 * 1/32 * 2 = 5/8. So overall, for a 6-game series, p = 5/16.

In order to have a 7-game series, games 1..6 must be split 3-3. The probability of this is 20 * 1/64 = 5/16.

The above calculations all rely on the binomial distribution. The factors of 2 are there to account for the possibility of either team winning. (That is, when I calculate the probability of going 3-0, it counts the probability of the NL team leading 3-0 plus the probability of the AL team leading 3-0.)

Comments: ( You must be logged in to post comments.)
  Subject Author Date
SolutionGeneral SolutionK Sengupta2007-06-19 05:05:13
AnswerK Sengupta2007-06-19 05:00:57
SolutionSolution...Erik O.2004-06-08 12:26:31
OOPS! (re: Additionally...)TomM2002-10-30 04:33:19
SolutionAdditionally...TomM2002-10-30 04:29:04
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